Configure Subscription Durations

[yonat]

I added support for storing the duration of a subscription product in MKStoreKitConfigs.plist, and later retrieving it using -subscriptionDurationForProduct:
This is useful for presenting the price per month in a shopfront (as recommended by Apple).

I updated he sample MKStoreKitConfigs.plist accordingly -- see my fork at https://gist.github.com/yonat/5f803b7ef84619711926