feat(version): generate version from git tags

This will generate the version based on git tags. So if it is built
directly on a tag it will use the tag name and if it is built off a
commit that isn't a tag it will look like:

    $ curl localhost:7001
    0-267-ga39cf1c
Merge pull request #59 from philips/generate-release-version

feat(version): generate version from git tags
fix(version): make the version numbers const

simple fix, thanks to @fmilo in #coreos
Merge pull request #60 from philips/const-the-versions

fix(version): make the version numbers const
use switch