Clean up the citation rendering so it doesn't display empty fields

Currently the citation rendering looks less than ideal when fields are omitted, like this:

Content created by [AUTHOR] of OpenStax College for [PROJECT], originally published at http://openstaxcollege.org/textbooks/introduction-to-sociology under a CC BY license.

Often times not all the information is available, so it is insufficient to suggest that those entering citations should just fill in all fields.

This pull request rewrites the citation rendering to adjust the citation display based on empty fields, creating a citation that only displays the information provided.