Fixes #116 - updates map markers

Updates map markers for better conformity with map clusters, and better
contrast with map appearance.

![screen shot 2014-05-12 at 2 37 30 am](https://cloud.githubusercontent.com/assets/704760/2942741/1eb2a6c4-d9b9-11e3-9100-e4ba9282d4d9.png)
![screen shot 2014-05-12 at 2 37 45 am](https://cloud.githubusercontent.com/assets/704760/2942743/1ec2fde4-d9b9-11e3-9b07-891bebde68af.png)

@alanjosephwilliams You might want to tweak these more in a PSD mockup. The grey circles don't have great contrast, and the clusters I think could be unified in the design better—at least by adjusting the color.