Stub initial adapter interfaces
Use NSCoding-like protocol names
Not actually JSON, but JSON dictionaries
Documentation for MTLKeyedArchiveAdapter
Documentation for MTLJSONAdapter
Actually, a keyed archive adapter is a horrible idea

There are useful methods on NSObject for object/class subtitution,
awakening, etc. An adapter pattern would just be mayhem.
Remove old MTLModel methods, split out <NSCoding>
Note that +modelVersion should be overridden
Remove old MTLModel methods from the implementation
Forward-declare MTLModel in MTLJSONAdapter
Implement MTLModel+NSCoding
Comment what a nil encodingBehavior does
Implement MTLJSONAdapter
Refactor value transformers to work only on JSON
Implement support for +<key>JSONTransformer methods
Declare old MTLModel methods as unavailable
Declare old NSValueTransformer methods as unavailable
Add MTLModel+OldArchiveSupport
Add an old MTLTestModel archive to the tests target
Refactor MTLTestModel to use new APIs