Add CLIENT_MODE option

AWS have a special memcached client that allows for autodiscovery. It
has an extra option OPT_CLIENT_MODE.

See http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/AutoDiscovery.html

Because the option handling code checks that the option is actually
defined before setting it, adding this option causes no issues for non
AWS clients.
Merge pull request #189 from bencoder/add-client-mode-option

Add CLIENT_MODE option