Different message on Windows after starting boot2docker VM

Docker client does not run on Windows for now. People have to SSH into
the VM to work on Windows.
Complain about VBoxManage not found only if necessary
More helpful advice for Windows users
Merge branch 'master' into fix/vbm-not-found

Conflicts:
	main.go
Merge pull request #39 from riobard/fix/docker-cli-win

Different message on Windows after starting boot2docker VM
Merge pull request #40 from riobard/fix/vbm-not-found

Complain about VBoxManage not found only if necessary