Memcached: memcached_gzip_flag directive.

This directive allows to test desired flag as returned by memcached and
sets Content-Encoding to gzip if one found.

This is reimplementation of patch by Tomash Brechko as available on
http://openhack.ru/.  It should be a bit more correct though (at least
I think so).  In particular, it doesn't try to detect if we are able to
gunzip data, but instead just sets correct Content-Encoding.