add github enterprise support

Nice application:) I'd like to add github enterprise support for this.

Added *Conf value to BaseAuth struct. Probably, Authenticator shouldn't have conf, 
Though It's very handy for support this feature. 

Seemingly, martini-contrib/oauth2 doesn't support github enterprise directly. At this time, I've wrapped NewOAuth2Provider function as well as `oauth2.Github`.

https://github.com/martini-contrib/oauth2/blob/master/oauth2.go#L109

Could you review my approach when you have a chance?
Thanks,