GeoJSON support

1. Add GeoJsonDataSource which as the name implies is a DataSource object which supports GeoJSON.  The only aspect of the specification that we do not currently support is holes in polygons.  This is a limitation of the DynamicScene layer which must be corrected first (I'll be writing an issue for it).
2. Hook it up to `viewerDragDropMixin` and the Cesium Viewer `source` query parameter.
3. Add a `ConstantProperty` which makes it easy to specify static values for DynamicScene objects.
4. Since GeoJSON does not include styling information, I provide user-modifiable default styling.  People with an eye for such things may want to check it out and offer suggestions.

![image](https://f.cloud.github.com/assets/1495196/684202/50ceeb8a-d9f2-11e2-957e-6ee7935605f5.png)