Use Docker binary and Vagrant box with Docker pre-installed

Much faster! Much simpler! :cake:

The Docker client binary is built using the current development version's cross-compilation support. This will make its way into 0.7.3 ( https://github.com/dotcloud/docker/pull/3337 ), so you might want to wait to merge this PR until we can use an official binary. In any case, I'm putting this PR here so we can at least discuss the change while we're waiting on Docker.

The Vagrant image is built from https://github.com/mitchellh/packer-ubuntu-12.04-docker . Maybe we can automate this somehow.

The other notable change is how upgrades work. I think it would be a good idea to force people to download a new version of docker-osx for each Docker release. That means we can keep docker-osx up-to-date and better fits the cognitive model of how docker-osx works (think: "I'm going to download the new version of Docker"). It also means we can create tagged releases of docker-osx for each version of Docker – we would be in lockstep with Docker releases.

Thoughts?