Implement support for sending the delete command

The delete command deletes a mailbox from the server. Add support for
sending the delete command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Implement support for sending the rename command

The rename command renames a mailbox to a new name. Add support for
sending the rename command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Implement support for sending subscribe commands

The subscribe command adds a mailbox to the server's set of subscribed
mailboxes. Add support for sending the subscribe command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Implement support for sending unsubscribe commands

The unsubscribe command removes a mailbox from the server's set of
subscribed mailboxes. Add support for sending the unsubscribe command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>