Child view controllers and main thread.

hey there,

i started using CDZQRScanningViewController because ZBarSDK was leaking hell and i fixed two bugs with this pull request:

* CDZQRScanningViewController was not usable as a child view controller with custom layout because the preview layers frame was only changed in willAnimateRotationToInterfaceOrientation. i moved the layout to viewDidLayoutSubviews.
* presenting an instance of CDZQRScanningViewController was really slow and blocked the main thread. thats why i moved the AV session initialization stuff onto a background thread. the view controller will now be presented immediately and have a black background until the camera input is available.