Added a better explanation for TestJWT and why it exists.

Author: mark-adams

tests/test_jwt.py

@@ -11,7 +11,12 @@ def utc_timestamp():
 
 
 class TestJWT(unittest.TestCase):
-
+    """
+    These tests exist primarily to ensure that calls to jwt.encode and
+    jwt.decode don't explode. Most functionality is tested by the PyJWT class
+    tests. This is primarily a sanity check to make sure we don't break the
+    public global functions.
+    """
     def setUp(self):  # noqa
         self.payload = {'iss': 'jeff', 'exp': utc_timestamp() + 15,
                         'claim': 'insanity'}