WIP: 1/2 make past events circles using images

This is my stub at #128 and second PR is coming.
![current_node_markers](https://cloud.githubusercontent.com/assets/1506905/4836477/0cbacc48-5fc5-11e4-8616-66559cba8156.png)
I will also take a stab at implementing the same functionality using [circle markers](https://www.mapbox.com/mapbox.js/api/v2.1.4/l-circlemarker/) instead of images. Though this one would work if we decide to go with something, which isn't circle (lolipop, maybe?).