Add comments to CallbackController methods
Drop build pipeline definition from wercker.yml

This pipeline is now added by the golang box by default.
Refactor GithubOAuthController

Grouped oauth properties in single service struct that is now part of
the controller.
Fix /users/callbacks returns 405

It seems that the /user/{id} handler got in the way. I moved the
/users/callbacks router registration to a point before /user/{id} to
give it a higher priority.