Correct ZXCapture setTorch:.

Fixes #95
Fix CGImageRef leak in ZXImage.

Fixes #96
Fix example apps
Start and stop capture sessions in background
Improve performance of BarcodeScanner

Initialize the scanner in viewDidLoad, and set/clear the delegate in the
appearance methods.
If ZXMultiFormatWriter is given a format it can't handle, set an NSError
Fixes a potential out of range error
Ported r2720 from ZXing

Original commit message: "Comment out unused encoder code (per Frank
Yellin)"
Merge pull request #1 from TheLevelUp/master

get update