[Android] BarcodeScanner - Upgraded to latest ZXing version

Had problems getting barcode scanning working on some test devices (e.g. Samsung Galaxy Tab 8.9, scan would work 1 out of 10 attempts). Upgrading to the lastest zxing-core and zxing-android from SVN fixed this.

Also added more activities to README.md which prevents the plugin from crashing when items on the menu are accessed.