Unsubscribe and resubscribe options

Allow for unsubscribe and resubscribe.

Adds Resubscribe value to fields array, which according to the docs when specified as true will resubscribe a user who is currently unsubscribed.

And adds the option to delete a user. When $_POST['campaignmonitor']['unsubscribe'] == 'yes' it will use a different api call.

Useful for when there's a profile page that allows for toggling between subscribed/unsubscribed