Added AFFetchSaveManager class to observe notifications and fire appropriate blocks

In response to some of the feedback in Pull Request #77, I created a manager class as a way to execute fetch requests or save requests, listen for the didFetchRemoteValues: and didSaveRemoteValues: notifications, and fire the appropriate completion blocks associated with that fetch or save request.

Let me know if you have any thoughts on this as a potential improvement on the current notification-based design. It's been working well for me so far.