Fix getuidvalidity crash (UIDVALIDITY returning None)

Rename getuidvalidity -> get_uidvalidity and cache the IMAP result.

1) Start modernizing our function names using more underscores
2) IMAPs implementation of get_uidvalidity was removing the UIDVALIDITY result
   from the imaplib2 result stack, so subsequent calls would return "None".
   As various functions can invoke this, this led to some errors that we
   avoid by caching the current UIDVALIDITY value in the Folder instance.

There are more simplifications and improvements to be made.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/Maildir.py

@@ -83,8 +83,10 @@ class MaildirFolder(BaseFolder):
         """Return the absolute file path to the Maildir folder (sans cur|new)"""
         return self._fullname
 
-    def getuidvalidity(self):
-        """Maildirs have no notion of uidvalidity, so we just return a magic
+    def get_uidvalidity(self):
+        """Retrieve the current connections UIDVALIDITY value
+
+        Maildirs have no notion of uidvalidity, so we just return a magic
         token."""
         return 42