Add checksum attribute for binary downloads

[bflad]

Release hashes: Every release of Docker now includes SHA256 and MD5 hashes of all build artifacts. These will be published on the documentation site and download page, allowing you to verify that your installation has not been tampered with. For example you can verify the SHA256 of the official Linux and Darwin binary builds with the following command:

curl https://get.docker.io/builds/Linux/x86_64/docker-0.10.0.sha256
curl https://get.docker.io/builds/Darwin/x86_64/docker-0.10.0.sha256

Do we want to be nice and add checksums automatically like this?
https://github.com/bflad/chef-stash/blob/master/attributes/default.rb#L12-L59