Fixed
- `get_options` of `ForeignKey`.
Added
- new fields: `JSON` and `ManyToMany`.
Updated
- renamed field `Url` to `URL`;
- renamed field method `get_metadata` to `get_options`;
- renamed field `default_error_messages` to `error_messages`;
- renamed `ForeignKey` parameters: `related_model` -> `to`, `related_field` -> `to_field`;
Removed
- support Tornado coroutines as defaut value for fields;