Configuration and overall enhancements

- Zoom factor added. Magnification is now calculated based on displayed image size instead of using natural image size as zoomed image.
- Background color configuration added for magnifying glass. This is useful when the lens overlaps other elements.
- Option to take page scroll in consideration for lens position added. Needed when placing the plugin inside a position fixed element.
- Max width and height parameters added. Very useful to resize displayed image keeping it's aspect ratio.