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).