dropdown events 'show' and 'hide'

Small patch that adds "show" and "hide" events on dropdowns. Useful for notification 
systems that track seen notifications. Related to issue #1343.

Usage:
```js
$('#myDropdown").on('show', function () {
  // code
})

$('#myDropdown').on('hide', function () {
  // code
})
```

Tested with Firefox 14.0.4 and Google Chrome 21.0.1180.81 on Ubuntu 12.04