V1 - rewrite Rocker from scratch, completely client-driven

[ybogdanov]

Rocker has been rewritten from scratch and now it became much more robust! While dockramp as a proof of concept of a client-driven Docker builder, Rocker is a full-featured implementation.

1. There are no context uploads and fallbacks to docker build. It makes your builds faster especially if you have a big project.
2. Cache lookup works much faster than Docker's implementation when you have thousands of layers.
3. Better output: rocker reports size for each produced layer, so you see which steps take space.
4. Works with Docker >= 1.8

What is not supported yet:

1. ADD <url>
2. Adding tar archives that supposed to automatically extract