Option to trigger build on closed pull request, build step to comment on pull request

I'd like to contribute the following:

- <b>Option to trigger build on closed pull request</b>: when we build a pull request, we have created database and deployed a test instance of our app to validate the fixes and to test the new features. The test instance of the app should be available until the pull request is merged/closed. So we need to be able to trigger a build when a pull request is closed to delete the database and the test instance as well as clean up any other resources used by the pull request.
![screen shot 2014-12-05 at 12 44 52 pm](https://cloud.githubusercontent.com/assets/7797160/5362108/39fb3e38-7f85-11e4-8cfd-310db8ace0ac.png)

- <b>Build step to comment on pull request</b>: until now I have to use curl to post comment using GitHub REST API, it'd be handy to have this build step.