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
No can do. I need the expected input type of `handlePullRequestEvent` to tell the type inference system that `parseEvent` needs to return a `PullRequestEvent` type. If I execute the function without that context it appears to infer the `PingEvent` type, not sure why exactly but it fails to compile the `handlePullRequestEvent` line since it thinks that `actualEvent` is a `PingEvent`. Next patch has an alternate approach.