Zoom affects width of county/metro boundaries

Per @ohsk's design, metro boundaries are now solid white lines. The boundaries for both county and metro also become wider as the user zooms in, though not at the same rate.

![screen shot 2014-08-06 at 9 21 26 pm](https://cloud.githubusercontent.com/assets/326918/3836291/9a56d69a-1dd1-11e4-9992-6ea713922478.png)
![screen shot 2014-08-06 at 9 23 57 pm](https://cloud.githubusercontent.com/assets/326918/3836292/9ea54ea2-1dd1-11e4-8eb6-edcbc131c653.png)
![screen shot 2014-08-06 at 9 24 11 pm](https://cloud.githubusercontent.com/assets/326918/3836293/a4046068-1dd1-11e4-8567-8e0c8fd8c690.png)