Set background-size for rescaled images. Fixes #14

Fixes this case:
- Plugin used directly on an image
- Browser needing the polyfill
- Image has been rescaled (width/height values different from the actual dimensions)
- Fading option is used

In this case the "colored" image shown on hover is actually a div with a background image. In order to preserve the scaling of the original image the div needs to have background-size set to the width/height values of the original image
Handle unproportionally rescaled images

If an image is rescaled unproportionally the svg ignores that unless the preserveAspectRatio attribute is set to none
Related  to 479e3ae
Merge pull request #17 from lichesser/fix_14

Fix for #14