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/accounts.py

@@ -378,7 +378,7 @@ def syncfolder(account, remotefolder, quick):
         statusfolder = statusrepos.getfolder(remotefolder.getvisiblename().\
                                              replace(remoterepos.getsep(),
                                                      statusrepos.getsep()))
-        if localfolder.getuidvalidity() == None:
+        if localfolder.get_uidvalidity() == None:
             # This is a new folder, so delete the status cache to be sure
             # we don't have a conflict.
             statusfolder.deletemessagelist()