Craft Hard Specification for Message.

[jalcine]

In order to ensure a decent level of predictability for (or a lack of the need of)
for messages in Wintermute, this issue (until the release of 0.1.0) serves as a
reference point for what a Message object should initially be in JSON. Things that
base themselves upon a Message like MethodCall and the likes will have their
own implementation but will be contained within a message.

Some things pivotal for a message's integrity:

• timestamp - The time, in UTC, at which a message was crafted. This doesn't
  represent when it was sent though, not sure if that matters.
• sender - The information needed to learn about the instance of Wintermute
  that sent this Message. Initially, this was equivalent to a JSON object
  representing a Module::Designation object but more information might be needed.
• receiver - The information needed to learn about the instance of Wintermute
  to receive this Message. Initially, this was equivalent to a JSON object
  representing a Module::Designation object but more information might be needed.
• payload - This would be the actual data being sent to the receiver. This
  is expected to be a dictionary, though later implementations might change this.

Proposed Fields

• flags - An array of text representing fields that were used to modify the
  payload value. Something like this would help with encryption, signing and the likes.

[jalcine]

#66 closed out the timestamp field. Now the sender + receiver fields can be done in tandem.