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.
Are you are suggesting changing all the factory methods in `Http2StreamException` (which currently also return Http2StreamException) and making the constructors non-public?  Otherwise this may put the onus on us to use the correct method.  The spec says this:

>RST_STREAM frames MUST be associated with a stream.  If a RST_STREAM
   frame is received with a stream identifier of 0x0, the recipient MUST
   treat this as a connection error (Section 5.4.1) of type
   PROTOCOL_ERROR.