This code will allow your app to connect to the app store's in app purchase service,
and to purchase products, and manage already purchased products.

I've also added some bells and whistles, like a progress bar for user feedback while the purchase
is in progress.

Unlike the StoreKit framework, this code will also work in the iOS simulator,
using the SimStoreKit simulator library.

CLONING

After cloning, you must initialize the git submodule:

$ git submodule init
$ git submodule update

NOTE:

Products that have already been purchased are stored in the device keychain.
For testing purchases, these entries are wiped whenever the test app
starts. Do not do this in production code!