look for prism and eprints metadata 

The Document plugin will now look for PRISM and Eprints metadata similar to what Google Scholar [does](http://www.google.com/intl/en/scholar/inclusion.html#indexing). This is in addition to the Highwire Press tags that it was already looking for. I renamed the 'scholar' metadata key to 'highwire' since it is more explicit that way. Google Scholar doesn't really have its own metadata format, and "citation_*" is from Highwire Press. 

The commit message should've mentioned that this fixes #212