contrib/github_buildbot.py Update

So I was playing around with github_buildbot.py to try and get it to work with my own project and after spending some time with it decided to open this pull request.  Here's a quick summary of the differences:

* Reject POSTs that are not a push event type
* Added support for the `X-Hub-Signature` header so messages can be signed by github and then verified on the receiving end
* Added responses to the incoming request so views don't return an empty body and so the request does not appear to timeout to github
* Added support for custom credentials to the perspective broker
* Better default for the --master flag
* Cleaned up logging and syntax in a few places