Add structure

ping @danizord @Ocramius 

Not finished yet, it's just boring as hell.

Questions are:

* I think provided mapping should only be a mapped superclass, as I expect everyone to extend the class (that's why it only contains the bare minimum).
* I still don't know where is the best place for updating password.
* Need to do password reset (not as easy as it seems, this thing is not very easy to map to REST)