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
I think you might be able to do something like this, using view patterns. Not sure if it's more readable or not:

    handlePullRequestEvent pev@(pullRequestEventAction -> PullRequestSynchronized) = handleUpdatedPullRequestEvent pev
    handlePullRequestEvent pev@(pullRequestEventAction -> PullRequestOpened) = handleUpdatedPullRequestEvent pev
    handlePullRequestEvent _ = return $ pack ""