Fix : #600

New pull request for #600.
Add <code>dropdownContainer</code> 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/579170/269870/fdab70d0-8fb0-11e2-8572-dda771baa328.png)