Support for GitHub Plugin

I added support for the [GitHub plugin](https://wiki.jenkins-ci.org/display/JENKINS/GitHub+Plugin) to enable links between Jenkins job pages and GitHub. The new syntax looks like this:

```groovy
job {
  name 'test'
  gitHubProject 'jenkinsci/job-dsl-plugin'
}
```

As always, I will adapt the wiki if you merge this.