Fixes #2584

 * Implemented instead as a public API, that can invalidate any report from any day or list of idsites
 * the API will be called by the Log Import script 
 * the archive.php next run will process these dates in priority
 * the ts_created is updated in the websites to make sure calendar is selectable
 * Handles when "Delete logs older than N days". Only invalidate reports that are more recent than N days (for which we are likely to still have logs)
 * Added integration test that 
  * first calls the reports, old data not displayed
  * then calls the API invalidating reports for newer dates
  * then calls the API again, now with data!

git-svn-id: http://dev.piwik.org/svn/trunk@6039 59fd770c-687e-43c8-a1e3-f5a4ff64c105