RENAME command - special INBOX

[EtienneBruines]

RENAME is now implemented (dev branch currently, but will be merged with master as soon as fetch is available), but requires some special attention w.r.t. the INBOX mailbox.

 Renaming INBOX is permitted, and has special behavior.  It moves
      all messages in INBOX to a new mailbox with the given name,
      leaving INBOX empty.  If the server implementation supports
      inferior hierarchical names of INBOX, these are unaffected by a
      rename of INBOX.

This would move all messages, and (which requires a bit more effort), update the counters (first unseen, recent count, etc.)