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)


is `room_id` being zero a Slack specific thing?