IE doesn't support overrideMimeType
IE doesn't support namespaces and getElementsByTagNameNS for XML, but it is still possible to retrieve namespaced nodes using getElementsByTagName with a 'theNamespace:theNodeName' argument.
FireFox needs a URI, but all browsers support the viz:* syntax as it turns out, so use that
Merge branch 'pr/89'

* pr/89:
  IE doesn't support overrideMimeType
minor change