Refactor DynamicScene vertex positions

1. Remove `DynamicObject.vertexPositions`
2. Add equivalent `positions` property to `DynamicPolygon`, `DynamicPolyline`, and `DynamicWall`.

This goes along with AnalyticalGraphicsInc/czml-writer#70

I was hoping to include support for polygon holes in this update, but that will now happen in a future pull.
The problem is the old code was wrong a lot of the time and any new code would be as well.  `positions` doesn't take extruded polygons into account, so if you had an ellipse in space you would zoom to the point on the ground.  In reality, I think we need a different approach for computing this altogether, so for now I just took this out.  If we have the time, I want to revisit it before 1.0