ref(Dockerfiles): use tagged common image based on ubuntu:14.04

This PR introduces a minimal **deis/common** Docker image as the base to all our components. `make build` will now also build `deis/common:v0.12.0-dev` (or whatever our version.go resource contains), and we will need to set up the Docker Hub to build the same. 

Closes #1561, deis/base#7, deis/base#13, deis/base#14.
Because [MAINTAINER](https://docs.docker.com/reference/builder/#maintainer) is optional, most production Dockerfiles I read seem to omit it, and until Docker gets its act together with efficient TCP transfers and parallelized downloads, every layer has a latency cost that is at best a couple seconds in my experience.

We can leave them in if you all would prefer.