Implement compatibility with the active_enum gem

This implements some compatibility with [Active Enum](https://github.com/adzap/active_enum). Also in the first commit this also implements some compatibility for [Simple Form's](https://github.com/plataformatec/simple_form) form field deduction when a field is inherited from the superclass.

The code probably isn't as clean as it can be; let me know how to improve. Thanks!