Add campfire output

I've added a campfire output. It sends events through to campfire which look like:

```
Riemann alert on gavin01 - test is OK - Description: Is this good
```

I've added some tests, though I wasn't able to test the actual campfire functions without mocking them out and that didn't seem ideal.

This is my first real attempt any any clojure beyond the config file, so apologies if it's full of silly mistakes!