Adding codeAsText to HttpResponseStatus.

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:
Adding codeAsText to HttpResponseStatus that returns the status code as
AsciiText.

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(HttpResponseStatus.OK.codeAsText());
Clean up 000d3a55c5b771b7ed8b71f54b44de72a85306c0

- Rename httpResponseStatus() to newStatus()
  - Move newStatus up so that static methods are grouped together
- Rename codeAsString to codeAsText