Added the ability to execute methods when the drop down is opened or closed

In this change; I added the ability to execute methods when the drop down is opened or closed.  It would be used something like this: 

<div     
    multi-select
    input-model="modernWebBrowsers"
    button-label="icon name"
    item-label="icon name maker"
    tick-property="ticked"
    on-Popupclose="onMultiSelectClose()" 
    on-Popupopen="onMultiSelectOpen()"
>
</div>

In the controller, you'd have code like this:

$scope.onMultiSelectClose = function onMultiSelectClose(){
   console.log('on multi-select close');
}

$scope.onMultiSelectOpen= function onMultiSelectOpen(){
   console.log('on multi-select open');
}