API changes & improvements

[romgrk]

So this PR:

• Changes this project's API a bit, to hide what's not required to know by API users (e.g. the protocol's internals)
• Adds drawing capabilities by including the lua cairo bindings. I've added the source inline rather than as a submodule because it was easier to setup and allowed me to quickfix the few required changes. This should be fine because as I mentionned elsewhere, cairo shouldn't be updated anymore.

This hasn't been reviewed/cleaned up by my because I've been quite busy but I've opened the PR so you can integrate if you're interested. Note that the README hasn't been update for the API changes. The examples folder shows how to use the new capabilities.