Adding timespan functionality for Top Searches block

This patch adds 'Number of days back to display searches for in top searches block' setting for Top Searches block.

It also adds cache table emptying after adding any new searches because I've noticed if I do not change default Drupal Performance settings (cache=Disabled, Lifetime=None) the block newer gets updated (the cron is called several times a day). If you do not want cache emptying just remove the line db_query("DELETE FROM {cache};");