Support for read-only calendars (closes #126)

* relevant RFCs go into the doc/ directory for reference purposes
* read-only calendar collections are set as read-only in Android
* HTTP exception refactoring to mark 4xx HTTP errors as hard sync errors (numAuthExcetions/numParseExceptions) for Android sync manager
* query current-user-privilege-set for resources, detect read-only resources
* show read-only resources as read-only in SelectCollectionsFragment
* minor refactoring (DavProp.*)