Use messages with __attribute__((deprecated))
Restore removed methods as deprecated instead
Default to JSON format for deprecated methods
Replace dynamic encoding behaviors with a dictionary

It's easy enough to override the actual encoding methods if more
specific behavior is needed, and this makes the 80% use case a lot more
declarative.
Split MTLTestModel into "old" and "new" classes
Update NSValueTransformer additions
Tests for the new MTLModel interface
Fill in encodingBehaviors dictionary by default

This makes it easier to override the default behaviors in code
(including by discarding any defaults entirely).
First pass at the new MTLModel implementation
Buncha fixes
Switch the default to MTLModelKeyedArchiveFormat

This only really affects backwards compatibility. New code doesn't have
a "default" format.