Refs #2327

 * we now check all websites that were last processed on a different day, and will process those. This ensures that even websites with no visits recently, will still have the week/month/year archives kept up to date.
   Use case: visit on Jan 5th/6th. Then no visit. Processing on Feb 1st: before it would not trigger January monthly archive, because there was no visit since last script run. Now it will trigger monthly archiving.
 * Added new API in SitesManager to fetch all websites which are set to specified timezones  +tests

git-svn-id: http://dev.piwik.org/svn/trunk@5822 59fd770c-687e-43c8-a1e3-f5a4ff64c105