Moved issue unpacking to BakerIssue

After download has finished, BakerIssue will take care of unpacking the files and updating the Newsstand icon and badge, to make the whole download process independent of any view controller.
Refactored download-related notifications

Each BakerIssue instance will now include its ID when posting notifications about download progress/finish/error.
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.
Optimised price retrieval for subscriptions