Periodic event basics, handle cancelled tasks

periodic events:
* functions for creation of periodic events
** string template substititon: insert date, time and custom fields
** works, but needs some more tuning for real world application
** respect status, just creates new events for every template atm

misc:
* helper decorator for custom field handling (like normal attribs)
* use UTC for "modified" time in ELSAEvent (Redmine uses UTC, too)