Adding support for parent modals

This patch adds a `data-parent-modal` property to the form which will automatically close the identified modal.

Example:

    <div class="modal hide" id="MyModal">
      <div class="modal-header">
	<button type="button" class="close" data-dismiss="modal">×</button>
	<h3>Edit Request</h3>
      </div>
      <div class="modal-body">
        <form
	 action="{% url edit_my_obj object.id %}"
	 method="post"
	 id="MyForm"
 	 class="form ajax"
 	 data-replace="#MyForm"
  	 data-refresh="#MyData"
	 data-parent-modal="#MyModal">
          ....
        </form>
    </div>

There is a catch. We don't want to close the modal if there are errors, so the view will need to provide an `errors` value in the returned JSON. Of course, this only applies if you are using the new `data-parent-modal` so backwards compatibility would be preserved.

    def form_invalid(self, form):
        data = {
            "html": render_to_string(
                                        self.get_template_names(),
                                        self.get_context_data(form=form),
                                        context_instance=RequestContext(self.request)
                                    ),
            "errors": True
        }
        return HttpResponse(json.dumps(data), mimetype="application/json")