Add support for SUBSCRIBE, UNSUBSCRIBE and LSUB

IMAP can subscribe to / unsubscribe from folders. Servers such as
Dovecot do not automatically subscribe to newly created mailboxes, you
must subscribe to them after creation.

subscribeBox - Subscribes to the specified box.
unsubcribeBox - Unsubscribes from the specified box.
getSubscribedBoxes - Issues a LSUB command to the server. Whilst LIST
returns all folders, LSUB only returns the folders the user has
subscribed to.
Merge branch 'subscribe' of https://github.com/Wismgari/node-imap into subscribe