Unit tests for pyscheme

Summary:
We finally have unit testing set up!

unittest works just like JUnit - subclass unittest.TestCase for your unit tests
and place them in a file in tests/. Methods prefixed with "test" are executed.

See python's unittest documentation at http://docs.python.org/library/unittest.html

Run all the unit tests with
``python -m unittest discover``

Sadly this functionality is only provided in Python 2.7+
I'll try to get a pre-2.7 solution at some point, given the popularity of 2.6
and its presence on the Berkeley servers.

Issue: #3

Test Plan:
Seriously. You want me to test a unit testing framework.
Fine.
``python -m unittest discover``

tests/__init__.py

@@ -0,0 +1 @@
+

tests/testprim.py

@@ -0,0 +1,16 @@
+import unittest
+
+import prim
+
+class PrimitiveSuccessTestCase(unittest.TestCase):
+
+    def testPlus(self):
+        self.assertEquals(prim.plus(1, 2, 3), 6)
+
+class PrimitiveFailureTestCase(unittest.TestCase):
+
+    def testPlus(self):
+        self.assertRaises(TypeError, prim.plus, 1, "two", 3)
+
+# import sys
+# unittest.main(sys.modules[__name__])