Build without docker and pull in third_party repos

This PR does two things to make building docker without docker installed first:

- Separates make.sh into common.sh, version.sh and make-without-docker.sh (thanks @mhennings [1])

- Adds a third_party directory that holds all external library dependencies for docker and adds a couple of scripts for managing it. The scripts are a bit rudimentary but have worked OK for etcd so far.

The relevant discussion on docker-dev is here: https://groups.google.com/forum/?hl=en#!topic/docker-dev/VeB_vmIMONc

[1] https://github.com/mhennings/docker/compare/build-without-docker