classes to eliminate issues with duplicated objects. closes#611, #612, #613, #618
* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.
* Updates to the Core Data layer such that NSManagedObjectContexts now have a reference to the managed object store
they belong to.
* NSManagedObject instances can now return the managed object store they belong to.
* Relaxed the coupling to the sharedManager present within the RKSearchableManagedObject class.
* Expanded documentation of RKSearchableManagedObject