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
Can we have phpdoc explaining what pattern and the other properties are? I have no idea what this does or what i should pass in by looking at it