Adding XML output to Python unittests.

gr_unittest.py was updated to handle the path and file creation and run the XML runner

gr_xmlrunner.py is a public domain program written by Sebastian Rittau <srittau@jroger.in-berlin.de> (THANKS!) to extend Python's unittest module to handle XML JUnit format output. This was renamed from xmlrunner.py if any future work is done on it from our project as well as to keep the namespace clean and clear.

qa_skiphead.py is a first test program for the XML output. Creates an outputfile $HOME/.gnuradio/unittest/python/test_skiphead.xml.