Rewrite to simplify things

I've rewritten the manager almost entirely taking into account all of iOS 6's new iCloud API.

Major improvements are:
- More consistent and transparent code
- Removed anything that didn't belong there
- Simplified the use of the manager as much as possible
- Minimized the public API of the manager
- Hopefully carefully handled all the dodgy cases
- Detecting user account changes or user deletion of the local container