Valera

- Add visit method for custom types [8](https://github.com/tsv1/valera/pull/8)
- Add `format_result` method [8](https://github.com/tsv1/valera/pull/8)

- Add `schema.datetime` type [7](https://github.com/tsv1/valera/pull/7)
- Add `schema.uuid4` type [6](https://github.com/tsv1/valera/pull/6)
- Add `schema.float.precision` [4](https://github.com/tsv1/valera/pull/4)

- Add type aliases [3](https://github.com/tsv1/valera/pull/3)