Give -[IssueManager refresh] a callback instead of a return value.
Switch IssueManager to use callback blocks in its refresh method.
Take network calls off the main thread.
Use of dispatch_get_current_queue() is recommended only for debugging. Use main queue instead.
Fix merge errors.
Merge branch 'master' of https://github.com/Simbul/baker into async-networking
Merge remote-tracking branch 'origin/master' into async-networking

Conflicts:
	BakerShelf/ShelfViewController.m
Add NKDontThrottleNewsstandContentNotifications constant so can receive more than one Newsstand download notification per day while in debug mode
Add semaphore so that magazines can be downloaded in the background
Move the semaphore so that it only gets used when app is in the background
Merge commit 'c370ac44fa492764ead4210b2875267dc5ed92b4' into newsstand_notification

Conflicts:
	BakerShelf/IssuesManager.m
	BakerShelf/ShelfViewController.m