ModelSerializer API.

Closes #1953.

---

To doc on `Serializer`
* `get_fields`
* `get_validators`

---

To doc on `ModelSerializer`

Determining set of field names:

* `get_field_names`
* `get_default_field_names`

Creating fields:

* `build_field`
* `build_standard_field`
* `build_relational_field`
* `build_nested_field`
* `build_propery_field`
* `build_unknown_field`
* `build_field_kwargs`

Determining extra field keyword arguments.

* `get_extra_kwargs`
* `get_uniqueness_extra_kwargs`

Validator API:

* `get_unique_together_validators`
* `get_unique_for_date_validators`

TODO: Clean up & define/doc the model info API