Add CSS support for GitHub like rendering

The pull request links directly to a gist for CSS. You can see how that gist visually improves the rendering of the Markdown. 

I think you might consider:

- (NSString *) css {
    return  @"<html>"
            "<head>"
            "  <meta charset='UTF-8'/>"
             " <link rel='stylesheet' href='MMMarkdown.css' type='text/css'/>"
             "</head>";
}

This would allow people the option of customizing their CSS by providing a CSS file in their resource bundle.  If the resource file is missing your library simply performs like it does today so the change to use a resource based CSS would be backward compatible in all projects.