Github webhook

Hello @tommoor,

In this PR you will find:
1. The controller action to handle the webhook posted by Github. Still need to implement some security with HMAC signature
2. Two methods on the Github Hook API to list and create a hook.
3. Tests for the Github adapter
4. Tests for the `IssuesController#github_webhook`

Figured out that would be easier to see how collaboration goes, I should have thought of it way sooner, but I was in the mindset of "wait 'til you got something showable".