Super rough first sketch of a Core Data adapter
Rename +modelClassesByPropertyKey to avoid conflicts
Stub methods
Move MTLManagedObjectAdapter into the main project

As long as we don't ever reference Core Data classes directly, we should be able to use methods from them.
Implement basic MTLModel > NSManagedObject serialization
Implement basic NSManagedObject > MTLModel serialization
Skip validation for relationships, serialize to-many relationships
Note that MTLModel to-many relationships only support arrays and sets
Implement deserialization of to-many relationships
Actually invoke +classForDeserializingManagedObject:
Set up some test models for Core Data
Merge branch 'master' into core-data-adapter
Tests for NSManagedObject > MTLModel
Avoid -performBlockAndWait: for confined contexts
Tests for MTLModel > NSManagedObject
Add a transformer for numberString
Track models created so far to avoid cycles
Add a property mapping for requiredString
Fix saving error
Track managed objects created so far, fix recursion