Add support for "Retina" sprites via the sprite-scale directive

These changes add a new sprite directive called 'sprite-scale' which enables smartsprites to generate sprites that, essentially, can increase the pixel density of images for high DPI displays like the Retina display.

For example, by specifying a sprite-scale of "2", and then supplying images in twice the size that you want them to display, smartsprites will generate a sprite--and accompanying CSS tags--that browsers will render at 2x pixel density (if supported by the browser/device).

While the sprite-scale directive accepts float values, the most common usage is likely to be "2", which is used for Apple's Retina displays.