Button that triggered the modal is now passed to 'shown' and 'show' as event.relatedTarget

When a modal is triggered by a button, the button is now passed as a relatedTarget to the 'shown' event as mentioned in #6159.

Also added show event as referenced in #6205

Ex:
```Javascript
$('#myModal').on('shown', function (e) {
  // e.relatedTarget is the element that triggered the modal
})
```

Tested on IE9, Firefox 14, and Chrome 23.

Resubmit of #6173.  I took Fat's suggestion:  The source button is now passed through event.relatedTarget.  

Now it is a resubmit of #6425 against 2.3.0-wip as per Fat's suggestion.  If there is something I have done wrong or a different branch to commit to please let me know :)  Thanks!