Update categories for iOS 6

- Also make resetting the categories consistent in both locations

Fixes #290
Remove resize mask from reader fullscreen image

Fixes #284
BarButtonItem was using wrong enum for type

Fixes #291
Updates to ContextManager:

- masterContext -> backgroundContext. The backgroundContext is used for general background tasks as well as the disk writer context.

- mainContext is still used for the UI

- newDerivedContext is still available for usage, for either background tasks or as a 'scratch pad'.

There is a crash (EXC_BAD_ACCESS code 2) on iOS 6 on mergeChangesFromContextDidSaveNotification:, but not on iOS 7 that we're working on.
Work-around for iOS 6 core data merging issue