Handle -version flag on all commands

Tested: Passed -version argument to kubelet (and all other binaries):
  $ output/go/bin/kubecfg -version
  Kubernetes version 0.1, build 6454a541fd56

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Improve generation of version information from the git tree

Detect whether the tree is dirty and append a "-dirty" indication to the
git commit (common practice with other repos, e.g. kernel, docker.)

Properly handle the case where a git tree is not found (e.g. building
from archive.)

In the sed expression, look for the variable to be updated
(commitFromGit) instead of hardcoding a line number.

Tested:

- Built from a dirty tree:
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build 2d784c684c75-dirty

- Built from a clean tree:
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build 505f23a31172

- Built from an archive:
    $ hack/build-go.sh
    WARNING: unable to find git commit, falling back to commitFromGit = `(none)`
    $ output/go/bin/kubelet -version
    Kubernetes version 0.1, build (none)

Signed-off-by: Filipe Brandenburger <filbranden@google.com>