Duplicate error messages in registration form

![screen shot 2013-12-06 at 4 10 38 am](https://f.cloud.github.com/assets/347918/1686589/5996ab06-5de9-11e3-867f-2562645ca662.png)

This is apparently caused by having both `:database_authenticable` and `:validatable` in `app/models/user.rb`. 

http://stackoverflow.com/questions/13897844/how-to-remove-duplicate-validation-errors
