A few spec compliance changes to pass the w3c tests

- relatedTarget must be null if pointerevent is capturing
- `new PointerEvent(..) instanceof PointerEvent === true`
- Manual tests: http://github.com/w3c/web-platform-tests

Manually verified by using w3c/web-platform-tests#324 and injecting PointerEvents polyfill into each test:
```bash
sed -i '' -e '/testharness.js"/{
a\
<script src="../../polymer/PointerEvents/pointerevents.js"></script>
}' web-platform-tests/pointerevents/*.html
```