Shade areas outside locked metro

When the map is locked to a metro area, set the style of all tracts outside it to a shade of gray. If viewing bubbles, do not add those associated with such outside tracts to the map (though keep them around for when the user unlocks).

This adds a global `lockState`, which is consulted when determining tract color as well as when adding bubbles. When the selector changes, tract styles are re-evaluated, and each bubble is checked to add/remove from the map.

Pictures!
![screen shot 2014-08-05 at 11 07 41 am](https://cloud.githubusercontent.com/assets/326918/3813379/ebadbb32-1cb2-11e4-84e6-f80ec0387e0e.png)
![screen shot 2014-08-05 at 11 07 57 am](https://cloud.githubusercontent.com/assets/326918/3813380/f11e0e32-1cb2-11e4-9ad1-3531d6627b7a.png)