Multiplexed

This continues on from my other pull request https://github.com/d11wtq/dockerpty/pull/1

Takes into account this thing docker does where we get 8 byte headers that we have to deal with.

(with thanks to https://github.com/dotcloud/docker-py/blob/a2f0f1b2ddbc166dcaa26ada67d94973aff7a9c4/docker/client.py#L255 for the inspiration)