Added support for IMAP move operation

[mpscholten]

This implement the IMAP move operation that is supported by some mail servers.

Usage:

-- Move email uid to mailbox "MyMailbox"
IMAP.move con uid "MyMailbox"

(While it might not be easy to test, I have used this function on my local fork in prod for a year)

[mpscholten]

@qnikst could you take a look at this as well? (and also the other PRs?) :)

[qnikst]

I've checked RFC, looks great! Thanks