`xunit.py`: Use `with...` to write report file
Added tests to improve coverage

There’s very little in the tests so far.  But hopefully now that files exist, it will be easier for others to add more vows as it occurs to them!
Bring `docstring_vows.py` up to date with refactorings
Refactored command-line parser into an `ArgumentParser` subclass
Added (minimal) tests to improve coverage
Merge pull request #82 from Zearin/feature/36-pyvows_test_coverage

Rewrote parsing as a subclass. Added tests for coverage.