Extract beats and notifiers modules.

I've extracted each `beat` and `notifier` to their individual modules. This makes testing each one of them in isolation easier; it also makes it simpler to add new beats and notifiers.

I ended up finding `heartbeat` after reading about (canary endpoints](http://byterot.blogspot.com.br/2014/11/health-endpoint-in-api-design-slippery-rest-api-design-canary-endpoint-hysterix-asp-net-web-api.html) and looking for existing implementations. If you have interest, I have some ideas on features/improvements to this project that I can post as github issues (and perhaps tagged as `feature` or `discussion`?).

Cheers! :beers: