Add support for auth with Corporate GitHubs

Minor changes needed to allow passport authorization with Corporate GitHubs plus the existing public GitHub already supported. Requires a new option (profileUrl) if you are using a corporate Github providing the URL for passport-github to use when doing the userProfile api call. For example, you might supply something like the following where corpDomain has the value for your corporate GitHub (e.g. github.mycorp.com)

"userProfileURL":"https://github.corpDomain/api/v3/user"

This is the only place that the api calling style differs for corporate  GitHub deployments for the things passport-github is doing.