Adding callback capabilities to loadJS

Hey gang! Ran into the need for a callback so I added a few lines. I didn't bother with <= IE8 as I'm using loadJS pretty much exclusively within a cutting the mustard/enhance check and IE8 doesn't make the cut. If you'd like, I can tack on a check for `script.readyState` to handle those browsers as well.

What say you? Any desire to have this in the function?