WIP: 2/2 add circle markers using divs

Just like #129, another stab at #128 using `L.divIcon`, which happens to be easier, that trying to work behing sheetsee's back & add own circle markers and bind popups. Attaching screenshot, though it's just like #129 
![div_node_markers](https://cloud.githubusercontent.com/assets/1506905/4836822/9a901d8e-5fca-11e4-9001-60aa098e66f9.png) though it looks nicer on extreme closeups.