Fixes memory leak due to waypoint retained nodes

Calling node.remove() before .waypoint('destroy') caused a memory leak
since waypoint uses jQuery data to store the node's waypoint ids and
.remove() clears all jQuery data. The result was that
.waypoint('remove') does nothing and the waypoint internals retain
every li ever created in the session.
Merge pull request #188 from robharper/fix/node-leak

Fixes memory leak due to waypoint retained nodes