Adding Http2ResponseStatus class with AsciiString statuses.

Motivation:
I found myself writing AsciiString constants in my code for
response statuses and thought that perhaps it might be nice to have
them defined by Netty instead.

Modifications:
Added Http2ResponseStatus class which creates AsciiString constants
using the code() from the various HttpResponseStatus constants.

In addition, added the 421 Misdirected Request response code from
https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#section-9.1.2

This response header was renamed in draft 15:
https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#appendix-A.1
But the code itself was not changed, and I thought using the latest would
be better.

Result:
It is now possible to specify a status like this:
new DefaultHttp2Headers().status(Http2ResponseStatus.OK);
make explicit? `.OK`?