Feature Request: Allow assertions on jQuery custom data

Howdy there, would be nice if I could have assertions on jQuery custom data attached to elements. For example

``` html
<div id="test" data-foo="bar"></div>
```

``` javascript
$expect("#test").to.have.data();
$expect("#test").to.have.data("foo");
$expect("#test").to.have.data("foo", "bar");
```

I imagine it's API would behave a lot like `.attr()`. Any thoughts?
