Add support for Slack

Added a new chat service and fixed some minor leftovers from the transition to string room ids.

Build notifications in slack:
![screen shot 2014-05-11 at 15 19 27](https://cloud.githubusercontent.com/assets/175675/3636035/73c84124-0f9b-11e4-8153-bc4a2f42458b.png)


I couldn't find anything on this at the moment, but I'm pretty sure I read anywhere, that in Campfire zero is a illegal room id and in [commit.rb](https://github.com/github/janky/pull/205/files#diff-2fe7ce19fc288c86bde365fbcd8c52d8L13) the room id gets check for being zero.

Should I remove this extra check?