Add wrap examples

Per https://github.com/Polymer/platform/issues/69
This adds some examples to demonstrate how and when to use wrap
This is a bad example since we can wrap methods of window and document and we have wrapped this one, and the most common ones already. The problematic ones are properties of window and document. Most common issue is `document.body` but also `document.documentElement`

```js
wrap(document.body)
// or get body of the wrapped document
wrap(document).body
wrap(document).head
wrap(document).documentElement
wrap(document).firstChild
```