Send mailbox names as modified UTF-7

The name of mailboxes should be sent to the server as modified UTF-7.
This is a fairly common operation and it is therefore useful to have a
method dedicated to sending such strings.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add method for sending the LIST command

Now when we have appropriate methods for encoding and sending as
modified UTF-7 we can implement a method that sends the LIST command to
the server. LIST requires that the name of the mailbox is encoded as
modified UTF-7. We can't however parse the replies yet.

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