b15 - 2013-04-01

Breaking changes:
  Billboard.computeScreenSpacePosition now takes Context and FrameState arguments instead of a UniformState argument.
  Removed clampToPixel property from BillboardCollection and LabelCollection. This options is no longer be needed due to overall LabelCollection visualization improvements.
  Removed Widgets/Dojo/CesiumWidget and replaced it with Widgets/CesiumWidget, which has no Dojo dependancies.
  destroyObject no longer deletes properties from the object being destroyed.
  darker.css files have been deleted and the darker theme is now the default style for widgets. The original theme is now known as lighter and is in corresponding lighter.css files.
  CSS class names have been standardized to avoid potential collisions. All widgets now follow the same pattern, cesium-<widget>-<className>.
  Removed view2D, view3D, and viewColumbus properties from CesiumViewerWidget. Use the sceneTransitioner property instead.
Added BoundingSphere.fromCornerPoints.
Added fromArray and distance functions to Cartesian2, Cartesian3, and Cartesian4.
Added DynamicPath.resolution property for setting the maximum step size, in seconds, to take when sampling a position for path visualization.
Added TileCoordinatesImageryProvider that renders imagery with tile X, Y, Level coordinates on the surface of the globe. This is mostly useful for debugging.
Added DynamicEllipse and DynamicObject.ellipse property to render CZML ellipses on the globe.
Added sampleTerrain function to sample the terrain height of a list of Cartographic positions.
Added DynamicObjectCollection.removeObject and handling of the new CZML delete property.
Imagery layers with an alpha of exactly 0.0 are no longer rendered. Previously these invisible layers were rendered normally, which was a waste of resources. Unlike the show property, imagery tiles in a layer with an alpha of 0.0 are still downloaded, so the layer will become visible more quickly when its alpha is increased.
Added onTransitionStart and onTransitionComplete events to SceneModeTransitioner.
Added SceneModePicker; a new widget for morphing between scene modes.
Added BaseLayerPicker; a new widget for switching among pre-configured base layer imagery providers.