We simulate resource sharing by decreasing weight.  The thing is if
weight is wrapped, that item continues to send DATA until its weight
gets lowered under the other items.  This commits fix this issue.

Since we have stream ID immediately available from nghttp2_submit_*,
we don't need adjust_priority_callback.

Previously stream ID was assigned just before HEADERS or PUSH_PROMISE
was serialized and nghttp2_submit_{request, headers, push_promise} did
not return stream ID.  The application has to check assigned stream ID
using before_frame_send_callback.  Now it is apparent that priority is
meant to DATA transfer only.  Also application can reorder the
requests if it wants. Therefore we can assign stream ID in
nghttp2_submit_* functions and return stream ID from them.  With this
change, now application does not have to check stream ID using
before_frame_send_callback and its code will be simplified.

timegm.h:30:8: error: C++ style comments are not allowed in ISO C90 [-Werror]

We inherited gzip compression API from spdylay codebase.  In spdylay,
the cost of having such API is almost free because spdylay requires
zlib for header compression.  nghttp2 no longer uses gzip to header
compression.  zlib dependency exists just for gzip compression API,
which is not an essential.  So we decided to move gzip code to under
src and remove zlib dependency from libnghttp2 itself.  As nghttp2
package, we depend on zlib to compile tools under src.

Also updates docs

Currently, nghttpd server only compresses files whose extensions are
one of .html, .js, .css and .txt.  nghttp advertises its support of
per-frame compression in SETTINGS frame.  To implement this feature,
we added 2 public API: nghttp2_session_get_remote_settings() and
nghttp2_gzip_inflate_finished().

This allows users of OS X 10.9 to run nghttpd (and friends) with
threading entirely disabled, to avoid crashes on startup related to
std::mutex.

libevent-server.c:552:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]

libevent-client.c:355:43: error: missing field 'recv_callback' initializer [-Werror,-Wmissing-field-initializers]

libevent-server.c:691:8: error: C++ style comments are not allowed in ISO C90