Use emoji for build status comments

One more! This one was more a just a little project to get to know the code, take it if you want it! I just replaced the success and failure icons with the analogous emoji from github: :white_check_mark: and :no_entry: 

Some niceties of github emoji instead of the markdown image syntax:
* Github auto-posts comments to our hipchat, and the markdown syntax was just getting posted in there without being parsed at all which was sort of ugly:
![image](https://f.cloud.github.com/assets/1906/1246641/1b5aedb4-2ab0-11e3-97b7-e200400a5ea9.png)


I imagine there are similar issues with other service hooks that don't parse markdown.
* Emoji get displayed in the github newsfeed
* No more hardcoded image paths back to your repo