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
Merge pull request #34 from sbonami/Add-ability-to-select-parse-class-name

Add ability to select parse class name