Cesium widget

I was expecting to open this pull request to get the discussion started on the new, easy-to-use widget, but this turned out so nice that I think it's ready for master (pending review of course).

1. New widget in `Widgets/CesiumWidget`.  It's dirt simple to use, lets you specify the base imagery provider, terrain, and clock.  Exposes the clock, scene, centralBody, a SceneTransitioner, and the DOM elements it creates.  It should provide a good base for other applications to be built on.  It's easy to use with other widgets, such as `Animation`, `Timeline`, and `Fullscreen`.  It has no viewModel because I don't think it warrants one; we can revisit that decision in the future if it proves problematic, or discuss it now if anyone feels it's important.
2. Deleted `Widgets/Dojo/CesiumWidget`.
3. Updated Sandcastle examples to use the new widget.  Some of  the existing Sandcastle examples can be further simplified using this new widget as well, such as the camera tutorial; but I'd rather see those done in separate pull requests.
4. This new widget is included in the single Cesium.js build.  I tested it by writing the simplest stand-alone skeleton I could.  The below skeleton.html will create a full-window Cesium application using the single Cesium.js approach. No require.js, no build, no dojo.  (This assumes the `Build\Cesium` output has been copied to the `Cesium` sub-directory of wherever you put skeleton.html.

```
<!DOCTYPE html>
<html lang="en">
<head>
<script type="text/javascript" src="Cesium/Cesium.js"></script>
</head>
<body>
	<style>
		@import url(Cesium/Widgets/CesiumWidget/CesiumWidget.css);
		body {
			padding: 0;
			margin: 0;
			overflow: hidden;
		}
	</style>
	<div id="cesiumContainer"></div>
	<script>
        var widget = new Cesium.CesiumWidget('cesiumContainer');
    </script>
</body>
</html>
```

Have at it and let me know what you think.