Extend version output

This MR is changeing the --version flag output to provide an extended version information:

# Before changes
$ gitlab-ci-multi-runner --version
gitlab-ci-multi-runner-linux-amd64 version 1.2.0~beta.71.gad1c29f (ad1c29f)

# After changes
$ gitlab-ci-multi-runner --version
Version:      1.2.0~beta.71.gad1c29f
Git revision: ad1c29f
GO version:   go1.5.3
Built:        Mon, 09 May 2016 17:16:51 +0200
OS/Arch:      linux/amd64

Closes #223 (closed)