The tests directory is now a subpackage.  This is more in line with
the recommended Python project file hierarchy as described here,
for example:

  http://as.ynchrono.us/2007/12/filesystem-structure-of-python-project_21.html

This should also make it easier to use the "use_2to3" with Distribute's test,
so that the directory containing the tests will also be converted to Python 3
when running `python setup.py test`.