add githubHook to rest ops

So we can keep the logic on our server not typesafe.com.
Otherwise we have to parse all the github stuff and figure out
what to do in the typesafe.com codebase which involves more work
to mess with.
Make github hook return the http status we should send to GitHub

This is a VERY roundabout thing to tunnel an http status through JSON,
but somehow we have arrived here.

Using a made-up HttpTextResult type to avoid depending on some
http lib just to send an integer and a string.
Merge pull request #11 from typesafehub/wip/havocp-github-hook

add githubHook to rest ops