Core Data adapter

A [MTLManagedObjectAdapter](https://github.com/github/Mantle/blob/core-data-adapter/MTLExtensions/MTLManagedObjectAdapter.h), for transforming `MTLModel` objects to and from `NSManagedObject`s.

**To do:**

 - [x] Serialize collections to `NSManagedObject` relationships
 - [x] Deserialize relationships to `MTLModel` collections
 - [x] Unit tests for serialization
 - [x] Unit tests for deserialization
 - [x] Check `+classForDeserializingManagedObject:` before deserialization
 - [x] Track objects serialized/deserialized so far to avoid cycles
 - [x] Document private `MTLManagedObjectAdapter` methods