GitHub: Some adjustments to method DoAuthenticatedGetRequest

- Instead of appending the access token to the query string, it is now specified using the Authorization header
- Added check to see whether the response stream is NULL
GitHub: Introduced class GitHubException

To foresee any future changes to the GitHub API, the class GitHubException has now been introduced as a base type for all GitHub related exceptions, so developers only have to catch a single exception type. However at the moment, GitHubHttpException is the only class to inherit from GitHubException.