Add support for github events and webhooks #256

I'm exposing a github webhook in a web service I'm working on. I'm using the github-webhook-handler, which uses github-types package, which is incomplete (it has 1 field implemented per event) and broken ("push" for example expects a "size" field, but the JSON payload doesn't have one).

This now effectively means one of three things:

1. I write a separate app in a different language with support for github events.
2. I handle the JSON directly (ew, no).
3. I improve support in either github-types or github. I already use github a lot, so I'd prefer this support be in github, but then I need to rip out the webhook handler and get it to work with github.

I don't have time for 2. and 3. I'm not sufficiently motivated to do 1. So I'll wait. I'm happy to provide sample inputs for each of the possible payloads.