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 think we would have to keep the `open(boolean halfClosed)` method. The `open()` method is for convenience for "normal" open scenarios.  It is equivalent to calling `open(false)`.  Is this OK?