added methods in User model for creating user with omniauth
added routes for sign in callback, failure and signout
edited the routes
added a sessions and identitites controller
added a helper method current_user in the application controller
added a new method for identities controller and new, destroy, failure methods to sessions controller
added a view new to identities
edited the application view to display messages
fixed the error in links in the pages index.html
typo