Fixes #53. Augmented the log data deletion feature. Added the ability to purge old reports and metrics.

Other changes:
  * Added the following plugin functions: Piwik_DropTables, Piwik_OptimizeTables, Piwik_DeleteAllRows. Also refactored existing code to use them.
  * Modified graph, tag cloud & datatable templates/views to show a different message if there's no data for a report and if its possible that report was purged.
  * Refactored DbStats API, added getAllTablesStatus method that doesn't modify the SHOW TABLE STATUS result.
  * Deletelogs config options are now stored in the DB.
  * Added task priority support to the TaskScheduler.



git-svn-id: http://dev.piwik.org/svn/trunk@6174 59fd770c-687e-43c8-a1e3-f5a4ff64c105