Map center update now happens on moveend instead of dragend

only using dragend is insufficient when the map has inertia movement on.
If the map is dragged with force, the center is updated to the final
resting place. Using moveend resolves this. This also removes the need
for the separate zoomend watcher.
Merge pull request #84 from grantlucas/grantlucas_update_center_move_end

Map center update now happens on moveend instead of dragend