Improved overlapping markers on map

Using Overlapping Marker Spiderfier
https://github.com/jawj/OverlappingMarkerSpiderfier-Leaflet

Produces:
For >= 12 murders:
![screenshot from 2014-12-24 18 55 07](https://cloud.githubusercontent.com/assets/44195/5547757/485a1b08-8bb4-11e4-88d0-cafc59db6807.png)

For < 12 murders:
![screenshot from 2014-12-24 18 57 05](https://cloud.githubusercontent.com/assets/44195/5547756/48593800-8bb4-11e4-9895-fd4a1559f0f7.png)