twitter oauth is working!!

 - remove avatar saving, since it used twitter's deprecated api
 - add a link to log in with twitter from the view (with pretty icon)
 - change twitter credentials to be pulled from environment rather than yml file
 - make email and password nullable in the database, since users created from
   the oauth flow will not have either
 - add provider column to o_auth_user which will become important when we get
   multiple services working