Added bootstro.on_complete(), bootstro.on_exit() and bootstro.on_step()

Added three new public methods that allow hooking functions to three bootstro events:
* Slide show complete
* Slide show stopped
* Slide changed (next or prev)

Now you can do this:
```javascript
    $(document).ready(function(){
        $("#demo").click(function(){
            bootstro.start();
        });

        bootstro.on_complete(
          function() {
            // last slide reached
            // close slide show automatically. no more annoying alert.
            bootstro.stop();
          }
        );

        bootstro.on_step(
          function() {
            // Slide changed.
            // Notify Google Analytics!
          }
        );

        bootstro.on_exit(
          function() {
            // slide show stopped.
            // focus() on registration button!
          }
        );
    });

````