The nghttp2_submit_{request,response} family do not get this change.

These test tools uses JSON as input/output and could be handy
to produce compressed header blocks.

Use huffman table in https://github.com/grmocg/httpbis-header-compression

This change conveys better notion about compression context.

We use huffman encoding only when its encoded length is strictly less
than the original length.
As noted in the HPACK draft, the length of name/value is now 7-bit
prefix and its MSB is 1 if the following string is huffman encoded.

So that we can preserve leading bits.

Conflicts:
	lib/nghttp2_hd.c

We wrongly added the whole payload length even if we sent part of it.

It turns out that 0-bit prefix is wrong, and the author now
clearly stated that the intention is 8-bit prefix for 8+ fields.

The current implementation uses Huffman code tables described
in http://tools.ietf.org/html/draft-rpeon-httpbis-header-compression-03

* remove substitution
* reversed insertion and removal from header table
* unified initial static table

Now shrinking local window size properly limits the amount of
WINDOW_UPDATE value so that shrinked window is honored.