Implement draft11

Here's the protocol-level implementation of draft11. All tests pass, new tests have been added where appropriate.

The one major change I didn't do is actually implementing the priority scheme on the connection (in other words, actually prioritizing streams). I felt that was an OK tradeoff to make, since the new prioritization scheme is still in flux, and all priorities are advisory, anyway :) Once we get prioritization more locked-in in the spec, I'll fix up the implementation of prioritization. Right now, this just means that all streams will have the same priority (or one less, for pushed streams).

I'll also be sending a PR to node-http2 to update the version of the protocol library used.