Logbuf rewrite

Closes #92.

Docker backend is untested, and logbuf event buffering until a newline is missing.
I'll change it back to the old codebase, basically I used the `Logs` call to retrieve old logs (if `req.Logs` because `Logs` is the only call that supports line counting, then I switched to `AttachToContainer`, since it offers us options that we cannot do with the `Logs` command (stdin, success callback, etc.)

Without that, we can pretty much fallback to just using the `AttachToContainer` option.