Provide herd as a download

[tombentley]

We should provide herd as an easy-to-use download, rather than requiring people who want to run their own version to compile it from source.

[FroMage]

So, a zip, a deb and an rpm.

[quintesse]

Isn't the zip simply: https://github.com/ceylon/ceylon-herd/archive/master.zip ?

(it's what I use for the Herd Docker image)