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)

Okay, I just pushed an updated version that works the same as yours.  We could probably create a general purpose function that takes an array that's something like `[<exact>, <word1>, <word2>]` and returns the best match, but that might be overkill and I didn't feel like editing more files for this PR.