Updates the plugin so that it can support multiple instances.

Multiple instances need to be applied to different elements in the DOM
otherwise multiple menus will be called for each click/right click.

e.g.

jQuery(document).ContextMenu(url);

would work for any form on a page. Using this would mean that multiple
instances couldn't be used though

jQuery('#content form').eq(0).ContextMenu(url);
jQuery('#content form').eq(3).ContextMenu(url);

Using the above 2 menus will be created for the first and 4th forms on
the page. Any of forms won't response to menu clicks.

jQuery(docuemnt).ContextMenu(url);
jQuery('#content form').eq(0).ContextMenu(url);

With the above any form on the page will respond to menu clicks but the
first form will send 2 requests for the context menu.