Add media_root attribute for sites served on multiple domains

[crsven]

Similar to pull request #10, but easier for the use case of someone serving media files from a different domain. Example: Apache serving pages at www.domain.com and Nginx serving media on media.domain.com.

By setting a media_root attribute, all CSS files will be processed using the media_root and HTML will be processed with the normal root.

Also included:

• Test for the media_root attribute. I didn't want to mess with your directory structure too much to provide a separate media_root folder, but if you have a suggestion for doing this better, let me know and I'll update it.
• Documentation update
• Updates to require statements in tests. I was having trouble running them from the root level on my machine. I think this should work more universally.
• A little whitespace cleanup :)