Feature - Model Equality

This PR contains two new methods for checking the equality of two ActiveRecord model instances.
It was born from a [comment][code-comment-origin] made to robinpowered/robin-apps-api#150.

I hope you like them!
:D

 [code-comment-origin]: https://github.com/robinpowered/robin-apps-api/pull/150/files#r10049072