devise with facebook login
google oauth2 login
better colors for fb and g+ buttons
back to root_path after omniauth login failure

`database_authenticatable` was removed from the user. As a conclusion
paths to login page are not added into routes. This has one drawback:
logout path need to be defined manually inside `routes.rb`.
login specs
better icons for facebook and google+
css corrected
Merge pull request #23 from Nuanda/devise

Devise login into facebook, google+

Fixes #15