Change event for input field

#1 commit:
I've added a new option "changeEvent" (boolean). If it's set to true, it will fire a JS change event on the input field, every time it gets updated by the slider.

#2 commit:
With this simple span tag in the handle element, you have more options for designing the handle with CSS.

A good example: If you would like to have a tiny handle but a bigger area to grab the handle, you just have to design the span tag and add a padding to the div.