Adding support for `select2` events

I needed to be able to fire actions when a tag was added and removed, if this functionality already existed I couldn't find it from the source.

To add events you simply need to add an `events` option to your select2 options object:

~~~javascript
$scope.options = {
    multiple: true
  , tags: ['one', 'two', 'three']
  , events: {
        'open' : function(e){
          console.log(e)
        }
     ,  'removing': function(e){
          console.log('removing a tag')
        }
    }
}