Adding in SUBSCRIBE, UNSUBSCRIBE and LSUB functionality

IMAP has a list of 'active' or 'subscribed' folders.
subscribeBox - adds a specified mailbox to the subscribed list
unsubcribeBox - removes a specified mailbox from the subscribed list
getSubscribedBoxes - same as getBoxes but instead of running the LIST
command, runs the LSUB command to get a list of only subscribed boxes.