allow specifying alternate repo url
include GOBIN in env
env package for easy manipulation of env vars
lock subdependencies
Symlink existing packages to tmp path instead of moving them

Also fixes issue where temporary package path already exists due to
Goopfile having multiple instances of subpackages with the same repo
root
show error message when a command fails with exit status
move all packages before installing
sort locked deps first before write so it's constant between iterations
Update README.md

Added Nitrous.IO jobs link.