Memcached store

### Problem
Rails 3.2.x `:mem_cache_store`'s implementation uses `memcache-client` internally, but we were able to pass a different client, which worked just fine.
Rails 4 `:mem_cache_store`'s implementation, however, follows only Dalli API, which breaks compatibility with `memcached` client.

### Solution
We can write a `memcached_store` that only follow `memcached` API. So we wont need to require `memcache-client` on rails3 or `dalli` on rails4 , and we can make sure our store works only with one memcache backend.

review @boourns @camilo @dylanahsmith @samuelkadolph @jduff