Don't require nose for tests

[cjerdonek]

If we can do test discovery without use of nose, then we should be able to run unit tests for Python 3 (issue #104) and pre-Python 2.4 (issue #103) more easily.

With regard to Python 3, my preliminary attempts to pass test_suite = 'nose.collector' to setup() for use with Distribute's python setup test seemed to have issues.

Specifically, this task is to set things up in accordance with the instructions here, namely to wrap the project's tests in a unittest test suite.