Update nose2/plugins/junitxml.py

Added some checked for cleaning up invalid unicode characters from output XML.
the xml cleanup patch was not Python 3 friendly
Moved XML cleanup functions to util.py.  This seems like a better idea.
Added tests and final cleanup
PEP 8 Cleanup
assertRegexpMathces is depreciated in 3.x and doesn't exist in unittest2 (2.6).
moved _xml_util.py back into junitxml.py
Organize imports