Add node autodiscovery

[jon-chard]

This package memcached-elasticache wraps memcached to support autodiscovery of nodes when its running in cluster mode.

Would be great if this package could do the same!

[saschat]

memjs is a binary Memcached client. Is there a reason you want to connect to Elasticache via the binary protocol? Why not use the client they recommend?