Uploading Images to Twitter as well as RetweetCount addition

Hi Daniel,

First, thanks for TweetSharp, it saved my bacon after the Twitterizer developer bailed.

I added some code to my fork of the TweetSharp library to allow it to post images to Twitter. If you've got a chance I'd appreciate it if you could take a look and let me know if I've implemented it in a way that is consistent with your thinking.