Release tgz and binaries for cross-compiled platforms

Fixes #1616

For examples, see:
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Linux/x86_64/docker-latest
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Linux/x86_64/docker-latest.tgz
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/x86_64/docker-latest
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/x86_64/docker-latest.tgz
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/i386/docker-latest
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/i386/docker-latest.tgz

Or:
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Linux/x86_64/docker-0.7.2-dev
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Linux/x86_64/docker-0.7.2-dev.tgz
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/x86_64/docker-0.7.2-dev
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/x86_64/docker-0.7.2-dev.tgz
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/i386/docker-0.7.2-dev
* http://tianon-docker.s3-website-us-west-2.amazonaws.com/builds/Darwin/i386/docker-0.7.2-dev.tgz