Minecraft Low Level API: First draft

Still WIP, I want to do a few tests before this is merged.

In the meantime, feel free to look over the code, I'd be happy about feedback.

I'm not sure how we should deal with the connection class. It's probably better if we keep it separate from the `mcpi` version, then we can improve it without having to deal with low level backwards compatibility (e.g. people trying to catch specific exceptions).

Once this is merged, it should fix #10.
Would `World` be a better name for this? It would make the API look like this:

    from minecraft import World

    mc = World()

What do you think?