Adding methods to turn a common NSUrl to a file based NSUrl and adding AppStoreReceiptUrl to NSBundle

The `AppStoreReceiptUrl` is the right way to access the in app purchases receipt for an application running in the sandbox in OSX (since the `transactionReceipt` is always `nil` for `SKTransactionPayment` objects in OSX).

The `NSURL` methods were missing from the current bindings, they are used to turn common `NSURL` objects into file based `NSURL` objects.