Use the viewfinder's view for calculating offset instead of full screen resolution

Fixes #19
This change uses the view's size when calculating the offsets for the view finder instead of the full screen resolution. This was causing an issue when using the barcode scanner in a view that does not fill the screen.
![resolved](https://cloud.githubusercontent.com/assets/1130691/3562371/d22c284c-09ed-11e4-9f50-b394a87c2392.png)