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
didn't work... not sure why... seems like it should

```
Handler/GithubEvent.hs:39:17:
    Couldn't match type `Data.ByteString.Internal.ByteString'
                  with `[Char]'
    Expected type: Maybe String
      Actual type: Maybe Data.ByteString.Internal.ByteString
    In the expression: lookup "X-GitHub-Event" $ requestHeaders req
    In an equation for `eventType':
        eventType req = lookup "X-GitHub-Event" $ requestHeaders req
```