Define custom events before and after page switch animations

After this commit we could define actions that run before or after changing the page.

```javascript
var page_selector = ".main";
$(page_selector).onepage_scroll({...});
$(document)
  .on('onepagescroll.animation.started', page_selector, function(ev, data){
    console.group('onepagescroll.animation.started');
    console.log(ev);
    console.log(data);
    console.log(arguments);
    console.groupEnd('onepagescroll.animation.started');  
  })
  .on('onepagescroll.animation.ends', page_selector, function(ev){
    console.group('#page-wrapper animation end');
    console.log(ev);
    console.log(arguments);
    console.groupEnd('#page-wrapper animation end');
  });
```