Implement unbinding events

For allow unbinding events, two things are needed:

1. Extend the plugin api to allow an __unbind__ handler

    (-- propose details soon --)

2. Make kss actually invoke unbind handlers when nodes are deleted or classes are changed.
    As there are no browser level events available to listen to, an explicit hook will need to
    be called to invoke unbinding / rebinding.

If this works some expected benefits will be:

- classes changed on the dom will cause events rebound

- deleted parts will have their events unbound, also a node:unload event can be
  properly implemented

- FireKiss will be able to toggle and change kss rules on the fly