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.
Maybe just call this StreamError ... seems a little strange that isStreamError() checks for instanceof StreamException.  I don't feel that strongly here, so feel free to leave it as-is.