Refactor categories (tags) section in posts a little

I turned the outer span/div tag (different in archive_post.html and
article.html) into a div tag that is conditional on the presence of
article categories.  Before this change, the outer div/span tag would be
present whether or not any categories were present; thus the little tag
icon would be displayed even when there were no categories.
Merge pull request #4 from L2G/refactor-category-tags

Refactor categories (tags) section in posts a little