Simplestyle

NOTE: Merge #2148  and #2161 before even looking at this (since the diff is overly large without them).

I would like to get this into 1.2, if possible.

* Added `GeoJson.fromUrl` to make it easy to add a data source in less code.
* Added support for the GeoJSON [simplestyle specification](https://github.com/mapbox/simplestyle-spec). [Sandcastle example](http://localhost:8080/Apps/Sandcastle/index.html?src=GeoJSON%20simplestyle.html);
* Added `PinBuilder` class for easy creation of map pins. [Sandcastle example](http://localhost:8080/Apps/Sandcastle/index.html?src=PinBuilder.html);
* Added `Color.brighten` and `Color.darken` to make it easy to brighten or darker a color instance.
* Changed default `GeoJSON` Point feature graphics to use `BillboardGraphics` with a blue map pin instead of color `PointGraphics`.

Here's what the pins generated by the new `PinBuilder` class look like:
![image](https://cloud.githubusercontent.com/assets/1495196/4452134/fb3e54b6-4841-11e4-8d7b-3554d6e98dda.png)

Typically, when darkening a color, one uses a multiply instead of a subtract.  This is so that colors like bright orange `(1.0, 0.5, 0.0)` when "darkened" by 0.5 become dark orange `(0.5, 0.25, 0.0)` instead of dark red `(0.5, 0.0, 0.0)`.

To brighten a color, you flip the effect over by subtracting from 1 before and after the multiply, like so:

```
    result.red = 1.0 - ((1.0 - this.red) * (1.0 - magnitude));
```