Hackathon polish

This adds various minor polish that we noticed when coding [Open Philly Globe](http://www.openphillyglobe.org/) at the hackathon.

* `CompositePrimitive.add` now returns the added primitive. This allows us to write more concise code.

```javascript
var p = new Primitive(/* ... */);
primitives.add(p);
return p;
```

becomes

```javascript
return primitives.add(new Primitive(/* ... */));
```

* Added more doc to the Polygon constructor.  Made some Sandcastle examples more concise by using this and the return of `primitives.add`.

* Turned WebGL validation off by default in the tests.  We'll use it when we need it and for releases.

@bagnell this will have minor merge conflicts with #1182.

@gbeatty you may want to cleanup your new picking example in #978 in the same way.