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.

Use NGHTTP2_ERR_INVALID_STREAM_STATE when trying to send DATA or
WINDOW_UPDATE to reserved stream.
Also a bit refactored nghttp2_session_predicate_window_update_send
and nghttp2_session_predicate_data_send

The NGHTTP2_ERR_PAUSE library error code is introduced to pause
the execution of nghttp2_session_mem_recv() when that error code
is returned from nghttp2_on_frame_recv_callback or
nghttp2_on_data_chunk_recv_callback. If this happens, the parameters
available for both callbacks are retained until the application
calls nghttp2_session_continue(). The application must retain
input bytes which was used to produce the frame.
After successful call of nghttp2_session_continue, the application
can continue to call nghttp2_session_mem_recv() to process
additional data.