added callback function support for on/off state change, so that user ca...

Callback support for for on & off state. When user click on/off button they may want to do some other thing too. To support this I have added callback support to on/off state.

on_callback, callback that will be executed after on state.
off_callback, callback that will be executed after off sate.

uses are given bellow,

('#switch').switchButton({
            on_callback: function() {                
                console.log('this will be executed after on state');
            },
            off_callback: function() {
                console.log('this will be executed after off state');
            },
            labels_placement: "left",
            on_label: 'ON',
            off_label: 'OFF'
});