Given the updates to the managed object store configuration patterns, it makes
much more sense for primary key configuration to be done against a mutable entity
rather than having the fall-back onto an associative object.
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.
