Allow omniauth-developer strategy in development

So, this is a first pass at a way to abstract authentication from the user so that we don't need to have a copy of auth  running on our local machines.

There's a few things I don't like:

1. I made provider/provider_id `attr_accessible`, which is a huge security flaw. Instead it would be better to make the Authentication model a real ActiveRecord model which keeps it's use and provider_id, and then make it's `user_id` attr_protected.
2. You have to type the provider and id in on the developer page. I've got to believe it's possible to pre-fill or add some 'default values' or something to omniauth-developer.