Implementation of the iOS AppStore purchase manager

This PR adds an implementation for the iOS Apple AppStore purchase manager. 

It still needs to be thoroughly tested, especially purchasing and SKPaymentTransaction to Transaction conversion.

On iOS 7+ it's necessary to start a SKReceiptRefreshRequest to get a receipt for verification. Perhaps we could add a StoreParameter for enabling this receipt fetching. The receipt fetching would then be done (if necessary) everytime before a transaction is finished.