Introduce semantic versioning.

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
Merge "Introduce semantic versioning."