GetInbox & GetInboxUnread

:+1: 
I'd rather we not implement the `ArrayAccess` interface here if it's all the same to you. If you really really *need* an array version of the data, you can always pass the instance to [`get_object_vars`](http://uk3.php.net/manual/en/function.get-object-vars.php) to turn it into an array. This `ArrayAccess` code seems like quite a lot for what it is, and it makes for a very different public API compared to the rest of the `Reddit\Thing` namespace.