Refactor DbHelper::truncateAllTables() to truncate non-core tables, and tweaks to DatabaseTestCase tests.

refs #6116

Adding new PivotByDimension DataTable filter that can pivot a report by (almost) any dimension. The filter can pivot reports by their subtable dimension and can also pivot by other dimensions (by using segments).

Notes:

- in the UI, only pivoting by subtable is supported
- change to CSV DataTable renderer so column names w/ commas & quotes can appear in text
- change to XML DataTable renderer so column names w/ invalid XML characters can be rendered (bit of an iffy change, XML format needs an overhaul I think)
- includes new config option 'pivot_by_filter_enable_fetch_by_segment'
- includes additions to component metadata classes (ie, Report/Dimension)

In IntegrationTestCase, create processed dir if it does not exist, add assertion to test individual API calls.

Fix git checkout command in travis.yml generator and correctly reference self:: instead of -> in IntegrationTestCase.php.

  - moved all API URL generation logic & API response handling logic to separate classes
  - created ApiTestConfig class to hold API test options (and documentation for properties)
  - remove unnecessary IntegrationTestCase properties and methods (including apiNotToCall/apiToCall)
  - remove as many unnecessary API test options as possible (including abandonedCarts, hackDeleteRangeArchivesBefore)
  - rename checkRequestResponse to assertApiResponseHasNoError

Refs #5800, add namespaces to integration tests in root folder. Also add IntegrationTestCase to a namespace, re-add individual test @groups, move test groups to class level, and rename test files & class names so underscores are not used.

When executing tests, check for non-core plugin that should be loaded by looking at test case class & current backtrace. (Removing need for enable_load_standalone_plugins_during_tests config option).

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source

refs #4706 added possibility to register a closure or a callable method to process processed metrics as late as possible and to make sure it always returns the correct value whenever it is called. Otherwise the value does not get updated in case for instance the nb_visits column changes (done in SumRow). Whenever there is a column that is not numeric and not the label column it should use a callback method instead of calculating the value directly to make sure the correct value is returend when using flat or truncate or ...

They pass locally.
CI-only build errors, my pleasure.

Add new console command to setup fixture outside of a test so manual testing can be done w/ existing fixtures and so specific custom data can be added by creating a fixture. Includes slight refactor to Fixture.php.

- Modify test user adding logic to hash password instead of using pre-hashed password.
- Allow testing environment to be configured via Option values that start w/ 'Tests.'.
- Put a couple test classes into namespaces. Add test event to Piwik\Mail class so emails can be intercepted.
- Change some test logging level to INFO.
- Added load + reload methods to PageFacade in capture.js