Marker Drag Events

ADDED: Two new Events are added to determine when the marker of a polygon-vertex is being dragged (start and stop).

From the readme:

// listen to when a marker of a polygon-vertex is being dragged
polygonLayer.on('pm:markerdragstart', function(e) {//...});
polygonLayer.on('pm:markerdragend', function(e) {//...});

FIXED: With active dragging, a single click on a polygon now doesn't trigger the pm:dragend and pm:edit events anymore (#84).