Support for unrecognised product IDs on restore

Transactions will be marked as finished even for unrecognised product IDs, but an alert will be shown to the user.
This is to avoid getting into a situation where the user is constantly asked to log into the App Store because of the unfinished transactions stuck in the queue.
Fixed crash with missing "creator" in book.json

The "creator" parameter should not be mandatory, but the app was crashing when it was not specified.
Fixes #768.
Example book.json "creator" is now a proper array