Doesn't work when targeting selects inside an iFrame in JQuery version

Issue #358 causes problems when trying to use chosen on a form contained within an iframe.

Since the form elements aren't directly accessible by selectors from the window object, the specific container_div isn't properly targeted.

This patch improves performance by not running an extra query against the dom to bind with the container_div

This bug only affects the JQuery version because the code makes an unnecessary search of the dom, which the prototype version avoids.