Ability to close currently open select2 dropdowns with a simple call

This is handy when you want to close all currently open select2 because of a stateChange (angular ui router) for instance.

The select2Stack factory can be used in the future to add other functionality.
Why not just make this a local variable? Don't think there's any need to keep it in an object field.

Will minify better if you put it in a local.

```js
var stack = [];

return {
    $register: function (callbackElem) {
        stack.push(callbackElem);
    },

    // Other methods here
};
```

Every byte counts :-)