Refactored App Store products callback logic
Basic auto-renewable subscription support
Added helper class to submit POST request
Added methods to retrieve purchases from server

The server needs to accept an array of product IDs and return a hash of product IDs and boolean values, representing whether an issue was purchased (true) or not (false).
Purchases from server are used to draw the UI
Added NSString category to generate UUIDs
Added generation of UUID to send to server
Fixed POST parameters for purchase pingback
Added return value for finishTransaction function

The function will return YES only if the app was successful in sending the receipt to the server.
Alert for error in sending receipt to server

If PURCHASE_CONFIRMATION_URL is specified and a purchase receipt cannot be sent to the server, an error message will be displayed.
Also, the transaction will not be marked as completed: the app will try to send it again next time it processes the transaction queue.
Send transaction type along with receipt

The transaction type can be 'free-subscription', 'auto-renewable-subscription' or 'issue' according to the product the transaction refers to.
Changed logic to determine when subscribed

The API call used by retrievePurchasesFor now has to return a JSON hash such as:
{
  "issues": {"product-id": true},
  "subscribed": true
}
Only show alert when actively subscribing

Transactions will be sent to the app for automatic subscription renewals: we don't want to show any message after we've processed those.
Added constants and clarified existing names
Added function to return app_id from bundle ID
Shelf JSON request sends app_id and user_id
Better error management for calls to PURCHASES_URL
Added message detailing subscription status
Merge branch 'master' into paid-subscriptions
Changes to conform to the new subscriptions API

Now the call does not require a list of issues anymore, and the issues in the response are an array (of purchased IDs) instead of a hash.