If max concurrent streams limit is reached, SYN_STREAM frames are not sent
and backed off. If other type of frame is waiting in the tx queue, it is
sent first. We introduced another priority queue for this purpose.
In this change we did not add code to send RST_STREAM when SYN_STREAM is
received but max concurrent stream is reached.

If we return negative error code in that function, it means frame will not
unpacked and ends up to lose any information the frame contains.
The spec says it should send RST_STREAM with PROTOCOL_ERROR, so we need
at least stream ID.
Therefore, the check should be performed in
spdylay_session_on_syn_stream_received().

It now always select "spdy/2" as a next protocol regardless whether or not
the server advertises it. The NPN draft allows this.
Returning integer version number is not flexible because the selected protcol
is just a string.
The function now returns 0 if the server advertised spdy/2, or -1.

We use just single buffer to store name/value headers fields, instead of
allocating memory for each name/value strings.
It is now more than 2 times faster than old one.

Specified stream_user_data_arg can be retrieved by
spdylay_session_get_stream_user_data() function. The application code can use
this function insde spdylay_on_ctrl_send_callback() and identify stream ID
for the request associated by the stream_user_data.
The sample usage is in examples/spdycat.cc.

Conflicts:
	examples/spdylay_ssl.cc
	tests/Makefile.am
	tests/main.c

Changes:
        spdylay_select_next_protocol() returns -1 if it fails.
        Use cunit without pkg-config because debian does not provide .pc file.
        Some doc updates to suite my taste.
        Added spdylay_npn.h

Closes all server-pushed streams when original stream is closed by RST_STREAM with CANCEL from client.

Fixed spdylay_session_is_my_stream_id()

With this function and 3 new member in spdylay_stream, we can track server-
pushed streams which associate them to this stream.

We still does not check "url" is in nv.

Minor updates to make git status work better

SETTINGS ID is little endian in wire format despite the fact that spec/2 says it is network byte order.