Allow automatic swapping in of img src from data-src when needed.

To allow lazy-loading of any image, add an API method swapInDataSrc that will copy the value from img-tag attribute data-src into src. This allows using a tiny placeholder image in the src, while still allowing preloading of the next image for easy transitions.  Note that this is convention-driven to expect markup like <img src="placeholder.jpg" data-src="realimage.png" />.  Any later DOM manipulation will work _just fine_ because we pull the data-src attribute (and delete it) once.  We could add the ability to specify the swapping attributes and such, but meh.