Made improvements to the script.

* `.active` class which used to be added when toggled were conflicting
with bootstrap class, thus changed to `.on`.
* Changed so that target element(s) should be specified via
`data-target` attribute (like how bootstrap collapse script does). This
allows multiple toggle parts on one page.
* `togglejs()` method is added to jQuery, allowing it to be triggered
like this:
$(this).togglejs();
* `togglejs()` can be used to trigger one-way animation:
$(this).togglejs('show');
this does show the element, and if it is already shown does nothing.