RSTwitterCallGetXAuthAccessToken

I put up some sample code for getting an OAuth access token when calling https://api.twitter.com/oauth/access_token (for xAuth clients).

I was having trouble getting this working, so I wrote a one-.m-file, easy-to-debug class so I could make it work. It’s not meant to be used as-is, but it might be helpful to other folks writing apps that talk to Twitter.