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.
@nmittler - I had the the `StreamException` class as a `private` internal class to `Http2Exception` but this method was the reason why I choose to exposed it.  I figured that this method should never be allowed to be called with a generic `Http2Exception` and it must always be a stream error (reset streams are not allowed to be sent for the connection stream).  WDYT?