utest: add loxigen directory to front of sys.path

Python prepepnds apt-get installed packages even before PYTHONPATH. This caused
the parser unit tests to fail on machines with an older version of pyparsing
installed even though it's included in the repo.
Merge into master from pull request #26:
utest: add loxigen directory to front of sys.path (https://github.com/floodlight/loxigen/pull/26)