Fixes #1184 Great patch by Julien Moumne to add Scheduled Task API in Piwik

 * possibilty to schedule daily/weekly/monthly tasks
 * tasks are executed via the crontab script for now (refs #1411 should be updated to trigger the tasks as well)
 * features the first use case: a Monthly OPTIMIZE TABLE statement ran on all piwik archive tables (to defragment the space after we run the DELETE statements)
 * Next candidates: PDF reports by email, custom Alerts
 * comes his very serious unit testing

git-svn-id: http://dev.piwik.org/svn/trunk@2648 59fd770c-687e-43c8-a1e3-f5a4ff64c105