Fix #51: New init method for subclasses

cc @dmethvin

This is necessary because https://github.com/jquery/jquery/commit/2f627494f28c08e5bba3c32d6c9d9167503322fc#diff-7eb52b366866677666470e019283c8eaL35 stopped passing an instance of `jQuery( document )` to `jQuery.fn.init`.