Prepare for tests.

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

{{{
python setup.py nosetests --with-coverage
}}}

setup.py

@@ -94,6 +94,8 @@ setup(
         ],
 
     install_requires = requires,
+    tests_require = requires + ['nose', 'coverage'],
+    test_suite = 'nose.collector',
 
     namespace_packages = ['rattail'],
     packages = find_packages(),