Fixes issue #642.
- Re-implemented the fix for issue #642
- Looking much better, but still seeing some weirdness in iOS 5.x (the Index View doesn't show up and Shelf View shows up with incorrect orientation)
- Seems to work in iOS 6.0 without any issues
Added method to refresh the current shelf and download a specific issue and used it as the notification_refresh_and_download handler
Updated AppDelegate to receive UIRemoteNotificationTypeNewsstandContentAvailability notifications
Implemented method to handle remote notification when the app is in foreground
Updated application:didFinishLaunchingWithOptions: to handle remote notification when the app is in background (WIP - method won't get call when the notification is sent)
Merge branch 'master' into newsstand_notification
Merge branch 'master' into newsstand_notification
Fixes issue #587. Apparently, self.interfaceOrientation is unreliable.
Fix for crash when releasing modal web view

Fixes #574:
1. Open shelf.
2. Read a downloaded book.
3. Click on a link in the book that opens up the modal web viewer.
4. Click Back on the modal web viewer to close it.
5. Without doing anything else, double tap on the book page to bring up the index and navigation.
6. Select "Back to Baker Shelf"
7. Crash
Added method to return latest issue
Refactored ShelfViewController init methods

The `init` method was never called and `initWithoutBooks` was pretty much doing what an `init` is supposed to do.
Consolidated ShelfManager methods in IssuesManager
Refactored IssuesManager into a singleton
Moved download management to BakerIssue

BakerIssue is now NSURLConnectionDownloadDelegate instead of IssueViewController, which means we can start and track downloads without having to instance a view controller.
Added badge when a new issue is downloaded

The badge will only be visible when a download was finished while the app was not open (e.g. in response to a push notification or because the user closed the app before download could finish).
Fixed iOS 6.1 warnings
Refactored App Store products callback logic
Basic auto-renewable subscription support
Added helper class to submit POST request