Get away from hellabot

[m-242]

The message struct wraps around hellabot.Message, which adds a pretty big dependency not only to the gateway, but also to every app.
We need to replace this struct with our own to keep our apps as lean as possible dependency wise.

[m-242]

We also need provide Marshal and Unmarshal functions to make serialization of this struct easy.

[m-242]

Hellabot is now isolated from apps completely.