added interface for dataTable to be able to define the expected type in a function, maybe it makes sense to add/remove some methods from the interface?

 * updating all variables/methods containing Referers
 * RENAMING custom segments definition, and custom scheduled reports definition, so that when they load "Referers" segment/report it still works as expected
 * RENAMING old archived data so that archived blobs are called Referrers
  * Reverting part of 0deb2bbb
  * Update integration tests generating PDF+HTML with images on my dev box (GD 2.1 and PHP 5.5)

Notes:
  - Fix bugs in DataTable class where getRowFromLabel would fail if summary row label is returned.
  - Fix regression caused by datatable visualization refactor (add_total_row property was effectively ignored).
  - Added x_axis_step_size property to jqplot graph.

Refs #4040, #4041, move AllColumns ViewDataTable & Goals ViewDataTable logic to HtmlTable visualization class & remove those classes. Revised the way visualizations default/override view properties. Cleaned up the ViewDataTable class a bit. Added base type for datatable visualizations.

renaming Unzip to Uncompress,
Renaming Tracker/Referer to Tracker/Referrer,
and few more

Refs #4040, converted UserCountry plugin to use display properties instead of ViewDataTable methods.

Refs #4059 Work in progress: Conversion to use Namespaces: Period*, Metrics, Segment, SegmentExpression, PluginsManager.
Removed some deprecated code.

Refs #4041, fix bug in displaying DBStats reports by redefining how columns_to_display view property defaults and allow unique visitors to be displayed on reports that have it, outside of period=day.

Removing feature: enable_archive_parents_of_datatable = 0 (main reason: there is no test, second reason: there is no public plugin using this feature AFAIK)
@Timo if you need this feature, let's put it back with some tests..
 Refs #2697

such as removing duplicate code and a lot of refactoring, the code is now much more readable!

Refactored archive data querying code. Removed tree-like data structure from Piwik_Archive, added memory optimizations for archiving (not storing archive processing instances), added optimization for querying data indexed by site + date, added ability to select multiple blobs/metrics at once (from different plugins), added optimization to Piwik_ArchiveProcessing_Period where numerics/records are selected all at once instead of one at a time.

Also added data access layer object for archive querying code, allowed anonymous function objects to be used as DataTable filters, and removed 'timestamp' DataTable metadata.

 * new segment 'siteSearchKeyword'
Fixes #3903, #3905:
 * adding few fields in the Live API output to accomodate getSuggestedValuesForSegment
 * renamed other fields for consistency with segment names
Fixes #3906:
 * new API: getSuggestedValuesForSegment which returns top suggested values for a particular segment. It uses the Live.getLastVisitsDetails API to fetch the most recently used values, and will show the most used values first

 * Adding tests for everything. The test case actually generates data for all segments so that VisitsSummary.get returns some data for each of the 47 segments being tested returns some data.
  How it works:
  * generate extended data in fixture
  * Tests (1) call getSuggestedValuesForSegment for each segment, check there is some data returned for each segment
  * get the first suggested value from the list,
  * Tests (2) call VisitsSummary.get with this segment value, eg. countryCode==ru.
    * I worked this way for all 47 segments until all tests had some data ==> now we know that all segments have been tested and that the auto suggest works for all segments. TDD FTW!

Refs #3465, fix regression for flattener filter where queued filters were applied on the flattened datatable.

 * bug fix: min/max generation time didn't work in nested data tables because the column aggregation operations were not passed to sub tables
 * ui improvement: very small values were shown as 0s. use three decimal places if value is smaller than 10ms (e.g. display 2ms as 0.002s instead of 0s)

Refs #1700
* Refactoring + adding missing parameters to other sumRow() calls
* Fixing tests on PHP 5.4 / 5.5

 * DataTable_Row::sumRow has a new parameter $aggregationOperations which allows treating columns as min or max (not only sum)
 * The aggregation operations can be set on any data table or passed to ArchiveProcessing_Period::archiveDataTable()
 * The Actions plugin uses the mechanism to aggregate the new metrics as min and max
 * The metrics are in the API output but not in processed reports
 * The min/max values are shown in the tooltip that appears when hovering the average generation times
 * Integration test updates

https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md#45-abstract-final-and-static

Converting Piwik core source files, PHP, JS, TPL, CSS

More info: http://piwik.org/participate/coding-standards/