New createGist operation and fix to authorization lookup for correct OAuth scopes

Created a new createGist operation for creating Gists via the GitHub API. Also fixed a problem in the authorization lookup whereby the current code base takes the first authorization returned but the API and uses it for all operation regardless of the requested scopes. So if a user has any existing authorizations on their GitHub account such as the default "gist" authorization, then it will never have the required scope and specifying the scope within Mule will have no affect.