Improved Slack integration with message attachments.

Updated the Slack integration to use attachments, as requested by them.
It now looks like this (much, much prettier):
![screen shot 2014-04-06 at 12 00 38](https://cloud.githubusercontent.com/assets/237493/2624994/b31fa856-bd72-11e3-94f6-cf07e883b23b.png)

Can be merged before #6573.