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.
Also see my request for clarification https://github.com/http2/http2-spec/issues/668.  This essentially boils down to [section-6.3](https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-6.3) stating:

>The PRIORITY frame always identifies a stream.

The spec already has the following:
> The PRIORITY frame can be sent on a stream in any state

[section-5.1](https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-5.1)
> Note that PRIORITY can
   be sent and received in any stream state.