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.
yes absolutely. I'm just thinking of a way to clean up some of this logic. Having a VERSION file would clean up the sed stuff to

```
VERSION = $(shell cat $(SELF_DIR)/VERSION)
```

And `version.go` would read from that file as well. Maybe the client too, though I'm not sure how that'd work when we package the client.