Prepare for tests.

This doesn't add any actual tests but it should pave the way for that.  Tests
may be run like so:

.. code-block:: sh

   python setup.py nosetests --with-coverage

setup.py

@@ -209,6 +209,8 @@ setup(
 
     install_requires = requires,
     extras_require = extras,
+    tests_require = requires + ['nose'],
+    test_suite = 'nose.collector',
 
     packages = find_packages(),
     include_package_data = True,