Fix : #600

New pull request for #600.
Add dropdownContainer with the possible values :
"body" - default - use body
"auto" - closest absolute
jquery/dom node - use as is

Works very well with IE8+, without any overlapping issue
![select2-ie8](https://f.cloud.github.com/assets/579170/269915/d8a475f0-8fb2-11e2-8a41-1c475924925d.png)