add css filter to desaturate mapquest tiles

CSS update to make data layer stand out better, fixes #2371

shaded before:
![tiles-before-shaded](https://cloud.githubusercontent.com/assets/209380/5571941/224dc372-8f5d-11e4-9e86-d06f5f5dc21c.png)

shaded after:
![tiles-after-shaded](https://cloud.githubusercontent.com/assets/209380/5571943/29fd91f6-8f5d-11e4-812e-27ab7d5440b9.png)

scaled before:
![tiles-before-scaled](https://cloud.githubusercontent.com/assets/209380/5571930/10eee098-8f5d-11e4-9fed-7374d115fb84.png)

scaled after:
![tiles-after-scaled](https://cloud.githubusercontent.com/assets/209380/5571934/186a93e4-8f5d-11e4-9e66-0a72740b7273.png)

Is this unbound in `map.destroy()` at the bottom? If not, could you apply this via some `map.on('load')` event (or whatever leaflet gives you)?