libnghttp2: Remove dependency to zlib

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.
Update doc