automatically set versionName from git tag

I'd like to make the versionName automatically set based on the release tags and git describe. It makes it a lot easier when running lots of debug/dev builds to tell what's what. Here's how I did it in another project: https://github.com/guardianproject/ripple/commit/23d061c1f43528f9de9c89d3e5ec3b797c07d5af

Related to this is the naming scheme of the git tags. It used to be purely the version number. @mvdan added a v prefix starting in version 0.95. I propose going back to the old standard where it was just the number. That works nicer with the getVersionName above, and it makes the gitlab UI happier. Right now, gitlab shows 0.95-alpha1 as the most recent tag: https://gitlab.com/fdroid/fdroidclient/tags

I'll happily write the script to convert the tags to remove the v, but it has to be run by @mvdan since the release tags are signed by his key.