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 agree I'm not too fond of the `Error` vs `Exception` terminology mixture.  I tend to think there is more justification for having the exception type have `Exception` in the name because that seems to be a java "convention".  I'll think about it a bit more.