Facebook login

Adds basic support for Omniauth login using Facebook. 

We still need to separate Omniauth authorization into a separate table to allow Users to authorize access to multiple social networks (or login or other interactions we'd like to support). 
It creates a login [here](https://github.com/edpaget/Panoptes/blob/facebook_login/app/models/user.rb#L32) since we need to have that to give them a URL.