Add memcached adapter

In order to support the `CAS` operations we will have to stop using the ActiveSupport Cache class and create our own adapter. This is the memcached adapter with the interface we will need for the CAS/ADD changes.

This PR doesn't affect any running code, it is just the adapter with tests, but so far nobody is calling it.

review @camilo @fbogsany 
cc @hornairs @tobi