Added price caching for issues

Prices will be stored by the ShelfStatus class on retrieval. Refreshing the shelf will also asynchronously refresh all prices.
Alert in case of error connecting to App Store
Preprocessor calls to support non-Newsstand app

The calls are used to remove references to Newsstand and In-App Purchases from the non-Newsstand version of Baker.
Fixed observer removal and added safeguard

A typo in an observer removal was causing "Purchase successful" popup for issues that had actually not been purchased.
This commit also adds a safeguard to avoid popups being shown for the wrong issue/product.