HTTP/2 Draft 16

Motivation:
HTTP/2 draft 16 has been released https://tools.ietf.org/html/draft-ietf-httpbis-http2-16.

Modifications:
The HTTP/2 codec should be updated to support draft 16.

Result:
HTTP/2 codec is draft 16 compliant.
I'm not sure if it makes sense to move the activation/addStream logic to the open methods...I hope we can because it seems like a good thing to keep the methods symmetric. I will keep it in mind when restructuring the `createStream(...)` method.