Add support for cascading classes

Hi Oliver. I needed support for cascading css classes so I wrote the code to do it. I'm not sure what might be an applicable unit test for this but my test case was the following markup:

```<html>
<head>
	<style>
		.foo .bar {
			font-size: 25pt;
		}
		
		.foo .bar .bing {
			color: green;
		}
		
		.foo .bing {
			font-weight: bold;
		}
		
		.bar .bing {
			text-decoration: underline;
		}
		.zing {
			color:orange;
		}
		.bar .zing {
			font-style:italic;
		}
	</style>
</head>
<body>
	<div class="foo">
		<div>
			<div class="bar">
				<div class="bing">
					me
					<span class="zing">ow</span>
				</div>
				<span style="color:red;">Me<span class="bing">ow</span></span>
			</div>
		</div>
	</div>
</body>
</html>
```
which yielded the following fabulous output:

![screen shot 2013-08-15 at 11 36 32 pm](https://f.cloud.github.com/assets/16900/973366/1a9f7476-0625-11e3-8d68-b03ad71209a0.png)