Adds cache storage adapter for authentication (implements Zend\Authentication\Storage\StorageInterface)

I needed a cache storage adapter which implements ```Zend\Authentication\Storage\StorageInterface``` for use with ```Zend\Authentication\AuthenticationService```, so I implemented this one. It's modeled directly after ```Zend\Authentication\Storage\Session``` in order to mimic that class's behavior when used with the aforementioned ```AuthenticationService``` class.