Why:
These methods are used internally for the functionality of the library
and are subject to change. Therefore shouldn't be part of the public
interface.
This PR:
- Moves the ```register_factory``` and ```factory_by_name``` methods to
the ```FactoryBot::Internal``` class.
- Deprecates the use of ```register_factory``` and ```factory_by_name```
from the ```FactoryBot``` module.
- Improve formatting of the specs
