add function for resubscribe API call

Response to my support question:

"""
You are correct in that there is a private API for resubscribing
customers. You can call using the exact same parameters as our
unsubscribe API (http://www.getvero.com/api/http/#unsubscribes)
the only difference being the endpoint:

POST https://api.getvero.com/api/v2/users/resubscribe
"""