Add Promise to Message so authors can verify messages arrived.

[Altreus]

We don't want to supply the bot's own messages to the message supply because ignoring your own messages is tedious boilerplate. So we can add a Promise to the created Message and keep it when the API sees the message come back.

We would need to know how Discord munge messages so we can recognise that it was the "same" one. Maybe we're given a message ID on POST?

[Altreus]

Also events!