Object classes and styles

This adds support for passing objects to the class and style attributes:

```jade
div(class={active: true, inactive: false} style={color: 'red', background: 'green'})
```

```html
<div class="active" style="color:red;background:green"></div>
```

The styles thing is largely for better parity with react-jade, but I also think it just makes more sense as a way of setting the style.  The classes change will make it way easier to add multiple conditional classes to an element.