Added property scanRect in order to allow the user to set the area to be scanned for a barcode.

Hi, I was looking for a way of setting a specific area where the ZXCapture object would look for barcodes.
I've added a property to set the scanRect area, which allows the capture to crop the image before trying to decode.
If not set or set to CGRectZero (default), the capture will still behave as before, trying to decode a barcode in the whole image.

Thanks.