expose the difference if dimensions are not the same

resemble.js

@@ -439,6 +439,7 @@ URL: https://github.com/Huddle/Resemble.js
 						data.isSameDimensions = true;
 					} else {
 						data.isSameDimensions = false;
+					        data.dimensionDifference = { width: images[0].width - images[1].width, height: images[0].height - images[1].height };
 					}
 
 					analyseImages( normalise(images[0],width, height), normalise(images[1],width, height), width, height);