Asyncronous interface

@lingyan 

First pass at making storage-lru use an async storage interface.  For backwards compatibility/sync interfaces, users can use the SyncWrapper (should I make this automatic?).
Same comments on as in https://github.com/yahoo/storage-lru/pull/10#discussion_r21877660

I'd love to have a clean interface for the returned asyncObject, without _getItem, _setItem.  Also the current way is actually modifying the syncObject thats passed in, which is not a good practice.