Enforce uniqueness in the database.

I have been using this sample app as a guide to get the ember.js auth
setup working, and I figured I'd send a few suggestions for the rails
side while I'm at it. Feel free to disregard issue completely.

From the [docs](https://github.com/rails/rails/blob/6e5ab6fccb55727ef2e32c30a77a48c595316cae/activerecord/lib/active_record/validations/uniqueness.rb#L156):

Using this validation method in conjunction with ActiveRecord::Validations#save does not guarantee the absence of duplicate record insertions, because uniqueness checks on the application level are inherently prone to race conditions.