Feature/login

@hab278 You can add a line here like I did to make user_attributes serializable before destroying it and then leave user model pristine:

https://github.com/NYULibraries/privileges/blob/506d95f253d194384d252ccda6c4e334b7924d0c/db/migrate/20141202185852_populate_user_fields_from_user_attributes.rb#L4