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).