Continue Go rewrite

With this change, `fig up` is finally starting to look like the real deal.  Woot

Still need to make volumes work.  Additionally there has been no implementation so far of `fig run`, `fig ps` etc.  which would be nice to have for posterity's sake and not too difficult to implement by using the Docker CLI directly (imported in `command.go`).

Also I need tests.  Hopefully `service.go` tests won't be too gnarly since you inject the API wrapper to a service (so they can share one if desired) .  We'll see.
Ah, I forgot this was hardcoded in here.  It should be gotten from environment variable `$DOCKER_HOST` or default to `unix:///var/run/docker.sock` if that's empty