Support sorting by GeoJSON feature property

E.g., this would sort features by their `magnitude` property ascending, putting elements with higher values on top:

``` html
<div class="map">
  <div class="layer quakes"
    data-url="..."
    data-type="geojson"
    data-sort-property="magnitude"
    data-sort-ascending="true">
  </div>
</div>
```
