Propagation fixes

Allow stacking propagation (Marker -> FeatureGroup -> MarkerClusterGroup)

Fix crashing if you get a mouseout event after a marker is removed from the map.
This can be reproduced (in chrome) by:
Mouse over a marker.
Remove marker from map.
Move mouse.

This happens with MarkerCluster if you mouse over a marker, mouse wheel zoom out (causing marker to become clustered and removed), move mouse.