Set application version at compile time

[muety]

@b3n4kh proposed a pretty cool way for managing version information by setting the version tag right at compile time, see #39 (comment). This way, we could get rid of version.txt.

I'd love to have this:

• By default, version is set to development or something
• If a Git tag (and a Git repo in the first place) is available and attached to the very latest commit, set version to that tag at compile time
• If a Git tag is available, but attached to a previous commit (other than HEAD), set application version to <latest-tag>-<latest-commit-hash>, e.g. 1.0.0-df44f7f

The third point is not a "must-have", if too hard to realize, though.

@b3n4kh Feel free to give it a go, if you want.