Prune trailing whitespaces from code and documentation

They are redundant in all pruned cases and sometimes even create some
problems, e.g., when one tries to jump through paragraphs in vi.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

offlineimap/folder/Base.py

@@ -210,7 +210,7 @@ class BaseFolder(object):
            If the backend CAN assign a new uid, but cannot find out what
            this UID is (as is the case with some IMAP servers), it
            returns 0 but DOES save the message.
-        
+
            IMAP backend should be the only one that can assign a new
            uid.
 
@@ -493,7 +493,7 @@ class BaseFolder(object):
                 continue #don't actually remove in a dryrun
             dstfolder.deletemessagesflags(uids, set(flag))
             statusfolder.deletemessagesflags(uids, set(flag))
-                
+
     def syncmessagesto(self, dstfolder, statusfolder):
         """Syncs messages in this folder to the destination dstfolder.
 
@@ -514,7 +514,7 @@ class BaseFolder(object):
          uids present (except for potential negative uids that couldn't
          be placed anywhere).
 
-        Pass3: Synchronize flag changes 
+        Pass3: Synchronize flag changes
          Compare flag mismatches in self with those in statusfolder. If
          msg has a valid UID and exists on dstfolder (has not e.g. been
          deleted there), sync the flag change to both dstfolder and