Added array mapping transformer and refactored JSON array transformer 

See #191

If this looks good I can add tests. All of the tests for `mtl_JSONArrayTransformerWithModelClass:` continue to work without any changes.

Some detail in the error key `NSLocalizedDescriptionKey ` returned on `mtl_JSONArrayTransformerWithModelClass:`'s failure is now lost but we could check for an error in the array mapping transformer and modify the error message.

Unfortunately the diff looks pretty crazy for adding a new method.