User Sign Up

Final piece to complete: [setupCurrentUser on sign in](https://github.com/gschorkopf/frolfr/commit/e87503151b748c13150fde73f6a028c039e7a946#diff-90557946fd2d72192769084779307fb4R18). @killpack thoughts on how to accomplish this? I need to use the pieces in the SessionController.

Will finish [#18]
I guess we could just serialize the errors hash as-is by adding `errors` to `UserSerializer`, and then we could maybe do some sneakiness to hook up errors to form fields in the same way SimpleForm does.