Rename TestMessage to ExampleMessage

When py.test is searching for tests it detects TestMessage as a
potential class with test cases because it has 'Test' in its name.
Because TestMessage contains a __init__ constructor py.test returns a
warning that it can't collect tests from it. Lets rename the class to
ExampleMessage to avoid this issue.

Author: jerrykan

test/test_multipart.py

@@ -20,7 +20,7 @@
 
 from roundup.mailgw import Message
 
-class TestMessage(Message):
+class ExampleMessage(Message):
     # A note on message/rfc822: The content of such an attachment is an
     # email with at least one header line. RFC2046 tells us: """   A
     # media type of "message/rfc822" indicates that the body contains an
@@ -160,7 +160,7 @@ def testMultipart(self):
         self.assert_(p is None)
 
     def TestExtraction(self, spec, expected):
-        self.assertEqual(TestMessage(spec).extract_content(), expected)
+        self.assertEqual(ExampleMessage(spec).extract_content(), expected)
 
     def testTextPlain(self):
         self.TestExtraction('text/plain', ('foo\n', []))