Improve MODS to RDF conversion in Institutional Repository plugin

[mjordan]

http://www.loc.gov/standards/mods/modsrdf-primer-2.html describes how to represent MODS in RDF. The link to the XSL stylesheet is broken; I've reported this to the Library of Congress and will incorporate it into the plugin when it becomes available.

[mjordan]

Instead of building a MODS->RDF conversion plugin, I have added a 'static' plugin that serves up full RDF XML files. The XML files are not generated on the fly, as doing so takes several seconds using the Library of Congress XSLT. Also, PHP does not support XSLT 2 natively, which is the version that the Library of Congress' stylesheet uses.