Adding support for creating scheduling objects for multiple VEVENTS

- beforeCreateFile now handles scheduling objects with multiple VEVENTs
- beforeCreateFile automatically creates ATTENDEE scheduling objects and scheduling messages in inbox
    - Updated database schema accordingly

Evert, 
There are still some TODOs that remain, but with this PR hopefully you can let me know whether or not you think this is the right way to go about things or it it clashes too much with how you envisioned SabreDAV.