Removed all traces of XML, OVA and OVF rel. code

* XML, legacy code, not maintained -- removed
* OVA, legacy code, not maintained -- removed
* OVF, legacy code, not maintained -- removed
* Removed dependency on Nokogiri
* XML parser and renderers will be re-implemented in the near
  future
Merge pull request #75 from arax/xml_ova_ovf_cleanup

Removed all traces of XML, OVA and OVF rel. code