Enable differing domains for OAuth 2 requests

[samsullivan]

This solves #313 by only prepending this._baseSite to URLs if they don't start with /https?:/. For example, Facebook uses differing domains for these requests:

• getAuthorizeUrl: https://facebook.com/dialog/oauth
• getAccessTokenUrl: https://graph.facebook.com/oauth/access_token

[samsullivan]

Maintainers: I didn't modify the constructor to make baseSite optional, because I wanted to keep my changes to a minimum. I'm happy to update this further, if you don't like this design.