Code cleanup.

This is an example plugin and it should serve as a good example :)

 * by default, send performance.timing.responseEnd - performance.timing.requestStart as generation time with the tracking request
 * if the performance.timing API is not supported by the browser, don't send the metric
 * new method disablePerformanceTracking()
 * new method setGenerationTimeMs(generationTime) to set the generation after measuring it on the server side 

added a little debugging output to datatable_manager.

CORE

 * formatting sub-second times
 * getColumn() method on data table array (in order to behave the same as the regular data table class)
 * data tables can store in their meta data, which columns are empty (this is used in order to dynamically hide the new "generation time" column)
 * ViewDataTable and Api.getProcessedReport act according to the empty column meta data


SCHEMA

 * new column custom_float_1 in log_link_visit_action
 * new version to apply the change


TRACKER

 * Piwik_Tracker::setGenerationTime
 * tracking parameter "generation_time_me"
 * value is stored in new custom_float_1 column
 * the log importer can handle a group "generation_time_micro" which can be used in a custom log format. _micro is used because apache can log the time in microseconds but piwik processes milliseconds.
 * note: extension of JS tracker still missing


ACTIONS PLUGIN

 * for pages and page titles, add new columns sum_time_generation and nb_hits_with_time_generation to the blob archives
 * if they are set, compute avg_time_generation on the API level. if not, remove the columns and mark them as empty in the data table meta data.
 * show new column "avg. generation time" in the pages and page titles reports


plus TESTS for everything

Hopefully fix test now.

refs #3734

Fixes #3734, added 'external backlinks' and 'referrer domains' stats to SEO widget via Majestic SEO API.

update JSON implementation from 2012-10-08 version of json2.js; applied D.C.'s changes thru-out, including the defense against window.prototype (which I think refers to var declarations shadowing window.prototype)

Fix bug where manually clicking the 'Check for updates' link in header_message.tpl would show the 'You are using the current version' message even if there is a new version.

Fixes #3131, add new auto-updating widget that displays the number of visitors, visits and actions in the last N minutes.

Notes:
  - Added visitors metric to result of Live.getCounters.

Fixes #3805, reverted change in log importer that looked for end-of-line after format regex match and modified format autodetection logic to pick the format based on whether the format matches and the number of groups returned in the match.

Notes:
  * Added several more tests to log importer tests.py. Added tests for checking format of log files w/ extra junk info on log lines. Added individual tests for parsing regex format.
  * Modified log files used in ImportLogs test, added extra junk info to end of some lines.
  * Fixed failing test in tests.py for the S3 log file format.