imbox: added folders() method to Imbox

I think it would be nice to have the list of folders on the Imbox class along
with the message query rather than on the ImapTransport class. However, I have
left the existing list_folders() method on the ImapTransport to avoid breaking
existing code that is using imbox.

imbox/__init__.py

@@ -58,3 +58,5 @@ class Imbox(object):
 
         return self.fetch_list(**kwargs)
         
+    def folders(self):
+        return self.connection.list()