Making StormpathUser it's own class!

It no longer inherits from AbstractBaseUser and PermissionsMixin.

This is for several reasons --

- AbstractBaseUser doensn't support our proper password fields.
- It's doing a lot of local-database specific stuff which we can't be using.
- It it doing permissions locally which is deceptively simple for users.

Right now I've ripped out all permissions support so this doesn't trick users
into thinking their permissions are properly synced to stormpath.  We'll have to
add this back in smartly in the future.