Extract NSManagedObject category

This commit extracts the NSManagedObject category so that the
pf_resourceIdentifier can be exposed along with the various key name
constants.

Conflicts:
	PFIncrementalStore.xcworkspace/contents.xcworkspacedata
Add logic to specify intended Parse Object

This commit add logic to specify the intended Parse Object with which
the IncrementalStore should sync.

Conflicts:
	PFIncrementalStore/PFIncrementalStore.m
Allow for Many-to-Many relationships
Add logic for imitating Parse Reserved User Object

This commit adds the PFReservedObject class, which subclasses
NSManagedObject and forwards methods to the related Parse Reserved
Object.
Add logic for accessing CoreData object from Parse Reserved User Object