[added] router.stop()

This commit adds a router.stop() method that may be used to prevent
a router from listening to further changes to location. This method
is also called automatically when a router component is removed from
the DOM.

Thanks to @gaearon and @rpflorence for the initial thrust to get this
functionality into the router. This work doesn't entirely supercede #606,
but duplicates much of the work done there.