Implement dry-run on Account() level

1) Set attribute self.dryrun depending on whether we are in dry-run mode.
2) Don't actually call hooks in --dry-run (just log what you would
   invoke
3) Don't write out the mbnames file in --dry-run mode.

Repository, and Folder levels still need to be protected in dry-run mode
as of now.

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

offlineimap/ui/UIBase.py

@@ -461,7 +461,10 @@ class UIBase(object):
     ################################################## Hooks
 
     def callhook(self, msg):
-        self.info(msg)
+        if self.dryrun:
+            self.info("[DRYRUN] {}".format(msg))
+        else:
+            self.info(msg)
 
     ################################################## Other