Ability to specify a containing element for the dropdown (minimal and easier way)

Fix : #556
Fix : #600

This patch fixes #556 as @zacharynicoll started with a cleaner solution. It brings a total isolation of select2 component.


This patch fixes #600 as well since developper this way : 
```javascript
$("#select").select2({
  dropdownContainer  : $("#select").closest(".modal")
});
```
Tested on FF, IE8, IE9, IE10