Fixed crashes, stretched camPreview if isSingleScan

application crashed on nexus 7 which have only front camera, because of Google's API method Camera.open() returned null for devices without back camera.
Also, if isSingleScan, we dont need "Scanning" text or scanButton, so i stretched the camPreview to the fullScreen.
Merge pull request #18 from Khromushkin/patch-2

Fixed crashes, stretched camPreview if isSingleScan