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
It seems like you could also use `sinkParser` from http://hackage.haskell.org/package/attoparsec-conduit-0.0.0/docs/Data-Conduit-Attoparsec.html, along with `rawRequestBody` from `https://hackage.haskell.org/package/yesod-core-1.2.19/docs/Yesod-Core-Handler.html` and `json` from `https://hackage.haskell.org/package/aeson-0.8.0.0/docs/Data-Aeson.html`. I'm not sure if that would be preferable.