Add convenience method that checks if reply is OK

We occasionally want to check if an instance of MXMIMAPReply is an OK.
We can do this by checking if the instance is a member of the
MXMIMAPOKReply class. This requires a bit of code that will be needed
everywhere where we want to do the check. Implement a convenience method
that does the check for us so that we only have to call that method.

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

IMAP has support for a command that upgrades a non-TLS connection to
TLS. We can support this feature by sending the appropriate command,
wait for the response and initiate a regular TLS handshake. This
requires a slightly different command handler since the result of the
entire operation is not a reply message but rather a confirmation of the
success of the operation itself.

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