Use Github Commit Status API

This uses Github's Commit Status API to update the commit status of the pull request on build completion, in addition to the existing behaviour of adding a comment. This adds a *status* (success/failure) to the pull request, along with a basic description (could be improved!) and a link back to the Jenkins build. 

Further information on [Github's blog post](https://github.com/blog/1227-commit-status-api). Example below, although currently the description text will be 'The Jenkins build passed`

![commit status api](https://github-images.s3.amazonaws.com/skitch/status-001-20120830-161640.jpg)