GeoJSON cleanup

1. Rewrite the `GeoJsonDataSource` Sandcastle example to show how to apply custom graphics.  This will serve as a reference for a new `DataSource` tutorial I'll write sometime soon.
2. Delete no longer used sample data.
3. Removed `defaultPoint`, `defaultLine`, and `defaultPolygon` from `GeoJsonDataSource` because they just aren't very useful.
4. Created a new CSS class `cesium-infoBox-defaultTable` that can be used by data sources to receive default infobox styling.  Made `GeoJsonDataSource` use this class.
5. Fixed several bugs I uncovered during this process and update specs to all pass after changes.