Added open panel callback

I have an implementation where I am animating the button element that opens the panelslider.  I'm not able to attach another onclick event because the panelslider prevents events propagation.  Since that seems like the best practice, I decided to add a callback option for when the panel opens.  I also updated the minified code using UglifyJS.  This is tested and working in Chrome 38.0.2125.104 on Mac.