beam + beam/data + dockerscript

This is a *neutral* branch introducing 3 low-level components in *pkg*. It also introduces a proof-of-concept tool called "beamsh". None of this code is used directly by docker: it is intended as a first step towards planned upgrades to the docker internals.

* ```pkg/beam```: a simple communication protocol inspired by unix pipelines and the microservice pattern

* ```pkg/beam/data```: the simplest possible format for structured messages

* ```pkg/dockerscript```: a shell-like syntax for interacting with a beam endpoint