Tests could validate against OAI-PMH.xsd

[jrochkind]

One or more tests could validate against an actual OAI-PMH.xsd schema.

http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd

This would be easier if #68 were done, cause nokogiri has XSD validating features.

It might be easier if we switched from Test::Unit to rspec, rspec's testing support features like re-usable specs and clearer ways to re-use logic in various ways might make it easier to validate various responses against the XSD.