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). 
I think my reasoning behind this line was to allow omniauth users who allow access their email to "reset" their password and generate a new password to allow themselves to login with a login/password or their facebook account.