Added a Devise Authenticator

You mention [here](https://github.com/simplabs/ember-simple-auth/issues/33) an interest in including a Devise Authenticator in core. If that interest still stands, I'd love to see this integrated.

For the most part, this pull is self-contained. It does make a soft change to the way the `login_controller_mixin` handles its authorize function, though. Since many Devise developers will want a `remember_me` checkbox, authorize now looks for one and passes on the value to the server. If the form doesn't have a `remember_me` field, it'll just ignore that request.

There are also tests and a new section in the documentation for Devise. Let me know if there's anything else you need!