metadata improvements

[colomet]

We are using your plugin as an example of ours, and we find some issues we would like to show you:
The top, our google schoolar metadata, and down yours.

• The title tag, e.g., citation_title or DC.title, must contain the title of the paper. Don't use it for the title of the journal or a book in which the paper was published, or for the name of your repository. This tag is required for inclusion in Google Scholar.
• The publication date tag, e.g., citation_publication_date or DC.issued, must contain the date of publication, i.e., the date that would normally be cited in references to this paper from other papers. Don't use it for the date of entry into the repository - that should go into citation_online_date instead. Provide full dates in the "2010/5/12" format if available; or a year alone otherwise. This tag is required for inclusion in Google Scholar.
• Also the pdf URL is empty.

https://scholar.google.com/intl/en/scholar/inclusion.html#indexing