Add ability to send message

https://www.pivotaltracker.com/story/show/78628040
I think a list of results would give the amount of detail we need.

 * `OK` (true)
 * `DELIVERY_FAILED` (false)
 * `INVALID_INPUT` (UnprocessableMessageException, user somehow entered invalid recipient or our body wasn't accepted)
 * `INVALID_ACCESS_KEY` (InvalidAccessKeyException)
 * `ERROR_UNKNOWN` (ApiRuntime-/ApiDomainException, we really need to investigate the logs to solve these issues)

If you agree, shall I put them in an "enum"? (`SendMessageResult::OK`)