Format non-link footer metadata like links

With certain document formats, we want to show metadata that doesn't link anywhere. This commit changes the specificity of the selector to target spans and moves the text decoration: none to just act on `<a>` tags.

Before:
![screen shot 2014-12-17 at 16 11 55](https://cloud.githubusercontent.com/assets/449004/5474199/78b754ee-8607-11e4-8589-1875ba179836.png)

After:
![screen shot 2014-12-17 at 16 11 50](https://cloud.githubusercontent.com/assets/449004/5474203/7ca59444-8607-11e4-82ff-efe44290c66f.png)

@jonnywyatt This bit of CSS is also used by https://github.com/alphagov/static/blob/master/app/views/govuk_component/document_footer.raw.html.erb#L48, so specifying the class isn't possible unless I start adding more classes.