Issue #282 Implemented Release Asset Uploads

This task was a little bit different from the usual, but I think I got it. The endpoint is `https://upload.github.com` instead of `https://api.github.com` which proved to be a little tricky. However it worked, see https://github.com/carlosmiranda/testrepo/releases/tag/blah. The `*.txt` files there were uploaded using the RtReleaseAssets#upload method.

I also created the ReleaseAsset and ReleaseAssets interfaces as I proposed in #282.