add support for older OME metadata

After trying to use this module on a ZVI file from 2011, I realized that the metadata reader only processed metadata from the 2013-06 version of the OME-XML schema. I needed to make the package extensible for a project I'm working on anyway, so I figured I'd submit a pull request in case you were interested in incorporating my changes. This new version passes all test_omexml nosetests (which I've changed to reflect the loss of the qnome function) on Python 2.7.6. Let me know how I can help!