Make flags a set rather than a list

As this is essentially what it is, a set of values. This allows as
to do set arithmetics to see, e.g. the intersection of 2 flag sets
rather than clunkily having to do:

for flag in newflags:
  if flag not in oldflags:
    oldflags.append(flag)

Also some more code documenting.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Gmail.py

@@ -21,7 +21,6 @@
 
 from IMAP import IMAPFolder
 from offlineimap import imaputil
-from copy import copy
 
 
 class GmailFolder(IMAPFolder):
@@ -45,7 +44,7 @@ class GmailFolder(IMAPFolder):
     def deletemessages_noconvert(self, uidlist):
         uidlist = [uid for uid in uidlist if uid in self.messagelist]
         if not len(uidlist):
-            return        
+            return
 
         if self.realdelete and not (self.getname() in self.real_delete_folders):
             # IMAP expunge is just "remove label" in this folder,