Added msgpack serializer support

We needed a better compact, binary serializer for memcached that worked across non-PHP languages.  msgpack has proven to be very fast and compact and results in significant savings.

See: http://msgpack.org/ & https://github.com/msgpack/msgpack-php/

While I was in there, I fixed the igbinary support, which silently said it was working but actually fell back to the default for serialization due to a typo (MEMCACHE instead of MEMCACHED). :)