fix(scanarea): ensure the whole camera input is used when not specifying a scan area

The default scan area of `CGRectZero` resulted in a rect of interest with size zero from the camera. This means that no code was scanned at all as the scanner was looking in a 0x0 area.

fixes #15
refactor(scanner area): set a default scanner area to be a bit more explicit
Merge pull request #18 from michaelochs/feature/#15-updateRectOfInterest_breaks_scan_capture

Feature/#15 update rect of interest breaks scan capture