HTTP/2 Draft 15

Motivation:
A new draft of the HTTP/2 spec has been released.

Modifications:
Make updates as defined in https://tools.ietf.org/html/draft-ietf-httpbis-http2-15

Result:
HTTP/2 codec is draft 15 compliant.
I like it.  W.R.T exposing the interface...I think we would need to do this if the user would need to see this and get the stream Id.  I'll think about it.