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 commit 'aa35cca492e19c399d9412323942911024925970'
Merge commit '67d10fdfed4a48e5268096a7b0b39aa3018d0f34'
Merge commit 'f04d2c5e33e497b42793702aefd8a511535f34f0'
Merge commit '09c074e17d03e552135fbc81703d09b5c8b8a234'