add gems omniauth-twitter
edit omniauth.rb
fix bug ":services"
edit services_controller.rb for login with twitter
add gem omniauth-google-oauth2
add new service 'google'
edit google path
add google authentication
add gems omniauth-github
add new provider: github
add new service_route 'github'; finish the login with other services
Merge branch 'login_with_github'
Merge with 'login_with_twitter' branch and resolve merge conflicts.