Fixes #2810. Refactored MultiSites plugin:

  * Created MultiSites API w/ getAll method that gets visit/action/revenue & related evolution data for all sites.
  * Modified MultiSites Controller to use aforementioned API.
  * Fixed bug I introduced in integration tests, method doTest_TwoVisitors_twoWebsites_differentDays shouldn't allow tests to skip the API.getProcessedReport test.
  * Added mergeChildren & related unit test to Piwik_DataTable_Array.
  * Added static methods to Piwik_Site to get site data using an ID. Using it avoids having to create a new Piwik_Site instance.
  * Modified ColumnCallbackAddColumnQuotient so it can be better extended and so it's possible to avoid processing rows if desired.



git-svn-id: http://dev.piwik.org/svn/trunk@5626 59fd770c-687e-43c8-a1e3-f5a4ff64c105