added HUC layer toggle, off by default

Map logic like this should go in Map service modules. I would probably do the same though if I didn't know about OpenLayers' ability to toggle layer visibility after they are added to the map. [Check it out](http://dev.openlayers.org/releases/OpenLayers-2.7/doc/apidocs/files/OpenLayers/Layer-js.html#OpenLayers.Layer.getVisibility)! Now you know too! Knowledge-power-high-five!

In OpenLayers you can have layer added to your map, but not visible to the user. With that in mind, I would discourage creation and removal of the layers from the map. Instead, I would make the huc layer's name a public facing member of the WaterBudgetMap Service, so that you can toggle the existing layer's visibility and keep all map logic in the map module:
```
var map = WaterBudgetMap.getMap();
var layer = map.getLayersByName(WaterBudgetMap.hucLayerName)[0];
var currentVisibility = layer.getVisibility();
layer.setVisibility(!currentVisibility);
```