Add github issue lookup

Closes #2

- Add ghost-issues script that uses Github public issue lookup API

Anytime someone says something with an issue number in it, slimer will output some info about the issue.  Also, if you specifically ask slimer about an issue in another repo it will give you info about it:

![Example](https://www.dropbox.com/s/ou2dwaiqe9c56bm/Screenshot%202013-11-01%2004.35.18.png)