GitHub: Some further work on the SocialHttpResponse class

... and marking SocialHttpRequest as obselete for now
GitHub: Updated method DoAuthenticatedGetRequest to return an instance of SocialHttpResponse

Since the GitHub API returns some information (eg. rate limiting) in the headers of the HTTP response, the method DoAuthenticatedGetRequest will now return an instance of SocialHttpResponse rather than the raw body.
GitHub: Updated GitHubHttpException to better reflect the API response
GitHub: Lots of changes (and additions) to the GitHub implementation