retrieveAll() Method

I've been using this library frequently, but I always need to hop in and add a retrieveAll() method so I can access all the cached data at once. It returns a `key => data` array of all cached objects (in the current cache), but can also return a `key => timestamp, expiration, data` array by setting the `$meta` argument to `true`. Hopefully others will find use for this as I did!