Complete a merge of the mimelib project and the Python cvs codebases

for the email package.  The former is now just a shell project that
has some extra files for packaging for independent use (e.g. setup.py
and README).

Added a compatibility layer so that the same API can be used in Python
2.1 and 2.2/2.3 with the major differences shuffled off into helper
modules (_compat21.py and _compat22.py).

Also bumped the package version number to 2.0.3 for some fixes to be
checked in momentarily.

Lib/email/Message.py

@@ -607,17 +607,11 @@ class Message:
                 newheaders.append((h, v))
         self._headers = newheaders
 
-    def walk(self):
-        """Walk over the message tree, yielding each subpart.
-
-        The walk is performed in depth-first order.  This method is a
-        generator.
-        """
-        yield self
-        if self.is_multipart():
-            for subpart in self.get_payload():
-                for subsubpart in subpart.walk():
-                    yield subsubpart
+    try:
+        from email._compat22 import walk
+    except SyntaxError:
+        # Must be using Python 2.1
+        from email._compat21 import walk
 
     def get_charsets(self, failobj=None):
         """Return a list containing the charset(s) used in this message.