Parse HelloRequest and ClientHello messages

Incomplete, do not merge.
I'm leaning more towards the "unnecessary" bit. The answer to why a HelloRequest class that doesn't have any attributes is, well, because [the RFC says so][1]. And why do we need a "parse" function that doesn't actually parse anything -- we probably don't. Maybe this is something we should worry about when we look at renegotiation.

I wish I had better answers, but I don't. I'm probably going to propose deleting this for now, and making the PR just about the ClientHello message.

[1]: http://tools.ietf.org/html/rfc5246#section-7.4.1.1