Optionally opt out of implicit mappings

This adds a `+useImplicitJSONMapping` class method to the `MTLJSONSerializing` protocol that allows classes to opt-out of implicit JSON mappings. Properties declared by superclasses are not affected.

Since not implementing `+useImplicitJSONMapping` is equivalent to opting into implicit mapping, existing classes are left unaffected.

Fixes github/Mantle#138