Added the event slider:hold

I've added an additional event to detect the mousedown event on the dragger. In addition I've put a bind example into demo.html (the console.log is commented out so that IE works correctly).

This works very well in conjunction with:

    https://github.com/jperelli/jquery-simple-slider/commit/3e72a5e1f5082638c95f9cfcac710f035508d68d

which offers the 'release' event.