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