GeoJSON defaults

__NOTE: This builds on top of #2255, so review/merge that first__

* Added the ability to specify global GeoJSON default styling.  This is done by introducing simplestyle-like static properties on GeoJsonDataSource.
* Added an options parameter to `GeoJsonDataSource.load`, `GeoJsonDataSource.loadUrl`, and `GeoJsonDataSource.fromUrl` that takes the same set of parameters as above in order to allow basic per-instance styling.
* Deprecated the `sourceUri` parameter to `GeoJsonDatasource.load`, use `options.sourceUri` instead.
* Updated the GeoJSON & TopoJSON sandcastle example to show some default styling.
* Refactored the entire GeoJSON test module to use `waitsForPromise`.  Also added tests for the new functionality.