Ship license file in the crate

[nullr0ute]

For distros it's generally required to have the license files shipped as part of the source so the license should be included in the yasna crate

[est31]

I suppose one needs to add the license files to the whitelist in Cargo.toml. PRs welcome!

[decathorpe]

This was addressed by #77 but isn't part of any published release yet.