Experiment - Run Octokit on Appveyor

Plotting and scheming in the OSS space: https://ci.appveyor.com/project/shiftkey/octokit-net

- [x] display build status in README
- [x] get unit and integration tests running
- [x] make all the tests pass woooooooo
- [x] no Windows 8 XAML installed on build server. whyyyyyy?
- [ ] setup Actual Proper Build Environment Account
- [ ] update correct URL

Fixes https://github.com/octokit/octokit.net/issues/118