Add 'resumeOnPause' option

The slideshow will pause indefinitely if the "pauseOnAction" flag is set to true. This option will resume the slideshow after it pauses due to user action.

An added option could be to allow one to define the delay before it starts up again.

This can also be done in the ```after()``` callback with the following code:

```javascript
    after: function(slider) {
      // if slider is paused due to user action
      // we should start it up again after some time
      if(!slider.playing) {
        setTimeout(function() {
          slider.play();
        }, slider.slideshowSpeed);
      }
    }
````