Keywords in article

[davidfarmer]

I took me a while to figure out how to add keywords to an article.
(I needed to read the xsl. Maybe the necessary documentation is in a pending pull request.)

Some things I noticed:

1. The sample article does not have keywords, and in fact it does not have a bibinfo element.

2. Leading and trailing white space should probably be stripped from each keyword.

3. In the HTML, the CSS should add a colon (or period) and space after the word "Keywords".
   (that word is in title tags, but there is nothing between the title and the first keyword).

4. If I build the pdf in the default style, the keywords look okay. But in the amsart style the
   keywords have two periods at the end: one in the LaTeX source, and (I guess) the other added
   by the style.