Relative url() paths preserved.

I was in a situation using v0.6.1 where I wanted to create documentation for a bower module that had css like the following

```
h1 { background: url(logo.gif); }
```

This rule was failing in the generated documentation because logo.gif's new location relative to the documentation had not been substituted for the url above.

This branch and accompanying unit tests cover this scenario. I realise that master has changed a lot since the release of v0.6.1 so I created a new branch from that release for this change. Hopefully you can use it as the basis of a v0.6.2 bugfix (presuming there are no regressions) while you continue to work on the next release.