added a "build" command

if you want to build a project like docker/swarm you can do the following now:

$ echo "swarm" > .godir
$ docker run --rm -it -v "$(pwd)":/usr/src/myapp -w /usr/src/myapp golang:1.3.3 go-wrapper build github.com/docker/swarm

and when it has finished you'll have the swarm binary in your PWD