trying to add defusedxml support... needs reviewing

defusedxml.lxml._etree seems to be useless. 

Anyway, not sure:
* if there is actually any security issues. For instance: does anything happen if one tries to convert a xml file containing a "billion laughs" exploit? (see http://docs.python.org/2.7/library/xml.html#xml-vulnerabilities)
* if there is such issues, how to solve them with defusedxml ?

Need to look deeper into this question; that's beyond my competence for the moment.