How to integrate AppHarbor with Slack?

问题

There does't seem to be an official integration a.t.m. How can I do it manually?

What I'm looking to achieve is:

1. Every time the build goes from PASSED to FAILED, a (sad) message should be posted on Slack
2. Every time the build goes from FAILED to PASSED, a different (happy!) message should be posted.

来源：https://stackoverflow.com/questions/28087223/how-to-integrate-appharbor-with-slack