map updates

@jueyang Here are my changes as we discussed. I'm not sure what meant when you said the OSM tiles don't look good, it looks pretty good to me, and all the lines and labels are there for free :-)
I also added the box explaining the circles that we have in the current version, and changed the circle outline to black to match the current version.
- Add legend explaining circles
- Use black circle outlines
- Use the OSM tiles instead of stamen

![screenshot from 2014-07-11 21 34 02](https://cloud.githubusercontent.com/assets/1614/3560632/eb3c0958-097d-11e4-848e-98adf4138b90.png)