Add new method to return unregistered timestamp when querying the feedback service. Rename 'devices' method to be clearer of what data it returns.

Hello

It's possible for a device to reregister for push for a given app but still be included in the unregistered device list from the feedback service. From [Apple's documentation] (https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/Chapters/CommunicatingWIthAPS.html#//apple_ref/doc/uid/TP40008194-CH101-SW1) "Use the timestamp to verify that the device tokens haven’t been reregistered since the feedback entry was generated." 

This pull request adds a method to get the list of unregistered devices from the feedback service along with the timestamp of when the device was unregistered so you can determine if the device should actually be removed from your database.

I've aliased the old method name so it will just work for anyone currently using this project.

Thanks