Allow selectors to start with `>` and `&` to match only children of the element being listened on

```html
<body>
  <div id="first">
    <div class="thing" id="a">
      <div class="thing" id="b"></div>
    </div>
  </div>
  <div id="second">
    <div class="thing"></div>
  </div>
</body>
```

```js
var del = Delegate(document.body.getElementById('first'));
del.on('click', '& .thing', ...); // will match elements with ID a and b
del.on('click', '> .thing', ...); // will only match element with ID a
```

Very happy to consider alternative syntaxes - or even to consider making `&` prefixed selectors the default behaviour (ie. *always* check if the element is a direct child of the root element).

Thoughts, @orangemug?