Introduce public calendar api [WIP]


We need a public api in ownCloud core to access available calendars.

The basic design of the api follows the contacts manager approach which is in place for quite some time now.

There are basically 3 pieces involved:
- the manager - holding all calendars
- the calendar - holding events and tasks
- the event/task

Please review the api design! THX

@karlitschek @MorrisJobke @georgehrke @Raydiation @PVince81 @schiesbn @butonic THX
Maybe call it isCalendarAvailable() or isCalendarRegistered() or something similar then :)