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.
Yes they clarified some exceptions in the spec.  I just changed the instances that were specifically called out in the spec but I wonder if we should be throwing frame size errors for most of the cases in this class?