This introduces semantic versioning (http://semver.org/) in Prometheus:
- A new VERSION file contains the semantic version string.
- The "tarball" target now includes versioning and build information in
the tarball name, like: "prometheus-0.1.0.linux-amd64.tar.gz".
- A new "release" target allows scp-ing the versioned tarball to a
remote machine (file server).
- A new "tag" target allows git-tagging the current revision with the
version specified in VERSION.
Change-Id: I1f19f38b9b317bfa9eb513754750df5a9c602d94
