Implement /me callback

After a user has been authenticated, the OAuth consumer may request additional information about the user. This callback has been implemented in the UsersController.
Implement missing method for users#me action

A new method has been added to find a User based on the token he sent with his request.
Implement single sign off

A single sign off mechanism has been implemented based in cookies.
Merge branch 'feature/oauth-integration' into develop