This addresses issue #105 by letting all tests be run using
Distribute's test, and in particular without using nose.
This paves the way for automatically running 2to3 when running
unit tests from Python 3, as described in issue #104.

This change causes 308 of the 314 total unit tests to be run (and
run successfully so) under Python 2.7.  This is a first attempt.
We still need to get the doctests included and running.

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`.

Also corrected/improved the spec test harness's handling of unicode
since simplejson handles unicode differently from json.

All tests pass with Python 2.4 (with simplejson-2.0.9).