Add jquery.ready hook after defining plugin, to avoid breakage with modernizr/yepnope

Created by: purcell

See https://gist.github.com/1258742 for an example, which results in the error:

"TypeError: Object has no method 'dropdown' at bootstrap-dropdown.js line 31"

It looks like in these circumstances, the functions hooked into jquery.ready get run immediately, and so applying the dropdown behavior to specific elements before defining the 'dropdown' method causes breakage.

-Steve