[WIP] Switch from HABTM to has_many through

![](http://25.media.tumblr.com/tumblr_lzapsizV8K1qg4blro1_500.gif)

This is just a first iteration, feedback are very welcome!

Original issue: https://github.com/EppO/rolify/issues/178

ToDo:

 - [ ] Add MongoDB compatibility **please HELP**
 - [ ] Fix all the other specs
 - [ ] Add more specs