Fixed leaked body click event in IE

This is a fix for https://github.com/ivaynberg/select2/issues/1058 which still occurs in version 3.4.3.

Here is an example with buggy version 3.4.3:
http://jsfiddle.net/nfAXH/
Only in IE, when clicking on the select2, a click on the body is leaked.

Here is the exact same example with the fix:
http://jsfiddle.net/zH4yY/
Now when clicking on the on the select2 in Internet Explorer, no click event is leaked anymore.

Why?
Because the "#select2-drop-mask" is moved inside the container (as you can see in this commit). And as of line 714
```
  this.container.on("click", killEvent);
```
any click on the container gets killed.