Rewrite the memcached client

* Update syntax to ES6
* Use `memcached` instead of `memcache`
* Fix restrictions where expirations weren't pushed forward on GET
* Fix a bug where we were unnecessarily bumping expirations on key search

Closes #201

lib/document_handler.js

@@ -123,7 +123,7 @@ DocumentHandler.prototype.chooseKey = function(callback) {
     } else {
       callback(key);
     }
-  });
+  }, true); // Don't bump expirations when key searching
 };
 
 DocumentHandler.prototype.acceptableKey = function() {