Switched to Leaflet for map vector feature display

Now utilizing leaflet for vector overlays instead of d3.  Also adds
mouseover/mouse click events on map.

Also coincidentally addresses and fixes #127 point 1).

Since the vector display has been switched to Leaflet, this will now
allow #101 to be addressed.
More efficient mechanism for adding features
Working marker cluster with menu selection
Re-enabled MultiPoint features

Mainly for presentational purposes.  Please note #133.
Merge pull request #135 from benjwadams/cluster_markers

Cluster point features