Added msgpack support for Memcached

Added `msgpack` support for MemcachedEngine. `ext-memcached` has to be compiled with msgpack in order for this to work properly. Without, the engine will throw an exception that the serializer is invalid.

More information about msgpack:
http://msgpack.org/

Msgpack exists in memcached as of today:
https://github.com/php-memcached-dev/php-memcached/pull/103

Earlier pull request has been pending for some months, but now it's finally official.