Add simple binding to (lib)memcached

Add a libmemcached binding to v8-jsshell, exposing methods for
set/get/deleting keys from a memcached server. All exposed on the
MemCache object on the global object --

   var options = {"SERVER": "localhost"};
   var mc = new MemCache.Connection(options);
   mc.set("record_1", {device: "/dev/null", offset: 2});
   var rec = mc.get("record_1");
   if (record_1.value)
       writeln(rec.value.device);

The binding is not built by default, use memcache=yes to enable when
invoking make(1).