Rewrite this tool in Python

[fingolfin]

In retrospect, that's what I should have used from the start, but it started out as a simple shell script and just grew..

Using Python would make a lot of things simpler (e.g. resolving issue #21); we already use it to parse JSON. The only thing we'd loose is the ability of the .release script in a package to mess with variables used by ReleaseTools. But AFAIK nobody is using this "feature" (misfeature?) right now.