Add ability to send message

[phavekes]

This issue is imported from pivotal - Originaly created at Sep 11, 2014 by Daan van Renterghem

Implement MessageService#send(Message), ensuring meaningful exceptions are thrown.

Throw an exception when the API doesn't respond with a 200-ish status or when the delivery status is "delivery_failed" (we only support one recipient).

https://www.messagebird.com/nl/developers#messaging-send
http://docs.guzzlephp.org/en/latest/testing.html#testing-guzzle-clients
https://github.com/SURFnet/suaas-ra/blob/025e2cf18050bf96efa55dd6c1b4818fdcd98042/src/Mollie/SMSBundle/SMS/Service.php

[phavekes]

Blocked while waiting for feedback @DRvanR (Reinier Kip - Sep 17, 2014)