Buildbox Service Integration

Hi, I've added integration with my CI product https://buildbox.io. I've matched my service with the GitLab CI service. I've shipped GitLab support to Buildbox today, which includes the endpoints necessary for the merge request status and badges. I needed access to merge requests events so I refactored the service hooks to include the events and included them in merge request changes. I hope that's OK.

Please let me know if theres anything I need to do to get this merged into master.

Thanks! Here are some images of how it all looks:

![image](https://cloud.githubusercontent.com/assets/25882/3965505/b2ac6578-2797-11e4-94f3-a5a7892db85a.png)

![image](https://cloud.githubusercontent.com/assets/25882/3965508/b737369a-2797-11e4-9a5f-70d164366d45.png)

![image](https://cloud.githubusercontent.com/assets/25882/3965510/ba34aec2-2797-11e4-8b04-1b95a53acc2c.png)

![image](https://cloud.githubusercontent.com/assets/25882/3965511/be24753a-2797-11e4-80c8-01aa63518aeb.png)
By default, service hooks only trigger on push events anyway. If you need to customize that further you can set it inside the `compose_service_hook` method as a method of `hook`. In this case I think you just need to remove this parameter since it seems to default to your desired behavior.