Changing to /Message/ Resource, supporting concat-SMS

In September 2013 Twilio deprecated the SMS resource an favour of a [Message][1] resource. The change to support this is very simple (a URL change). Using this resource Twilio now support messages up to 1600 characters using Concat SMS. I have changed the `body.size()` validation to reflect this.

 [1]: https://www.twilio.com/docs/api/rest/message