Allow client creation from authorizing HTTP client

[CodyDWJones]

This change allows apps to supply a http.Client they already have, rather than relying on the Authenticator helper. An app might use common code to obtain an auth'd http.Client because it interacts with many OAuth services.

[CodyDWJones]

I didn't realize #107 had been submitted. Either approach satisfies my requirement. #107 is more future proof (if a non-exported field is added to Client) but until then quite a bit more verbose.

[zmb3]

Thanks for this, #107 is probably a nicer approach, but this is 100% backwards compatible so I'll merge this and we'll keep #107 around in case somebody wants to work on a v2 of this library.