added "draw:vertex_moved", "draw:vertex_removed", "draw:vertex_added" ev...

I needed google.maps (api v3) equivalent of 'insert_at', 'remove_at', 'set_at' events for polylines. I added "draw:vertex_added", "draw:vertex_removed", "draw:vertex_moved" events to Leaflet.draw.

I'm not sure if I did it correctly, but it works for the project I'm working for. Any comments are welcome!