Add github event handler

Note this just updates the head history table for now. I'll update it to actually display the history later.

@cpennington
yeah added the extension.

I think I misunderstood your suggestion. I was thinking that extension would implicitly convert ByteString -> String, it sounds like it won't do that. Instead it will implicitly convert the strings "pull_request" and "ping" into ByteStrings so that they can be used to match against the return value of the function.

I'll try that out.