Create HTML landing pages for articles

The landing pages are created by a plugin, `article_html_pages.rb` using the metadata in `_config.yaml`.  

For each article, there is now an HTML page at `archives/<issue>/<slug>`, so that stripping the `.pdf` from the article links will give the HTML versions.  These pages contain the citation information in plain text, and in the HTML head `meta` elements in Dublin Core and Google / Highwire notation.  

I did notice a few minor inconsistencies in the `_config.yaml`: only the latest issue has a volume number assigned, and the formatting for 2013-1 issue is slightly different as well.  Not really a problem, only that volume number is consequently not available for those entries.  

I did not change any of the existing index files, so these still point to the pdfs directly as they did before, and not to the HTML landing pages.  Not sure if you want to leave it that way or direct them to the HTML.  

- Carl

p.s. The whole idea of doing a pull request for a journal is awesome by the way.  Don't want to make more work for you, but might be nice to share a short comment about R-journal "open for pull requests" on my blog, if that's all right?