Version return is missing

bcbio_vm.py doesn't appear to currently support the return of version information. Only the `--help` flag is supported:

```sh
bcbio_vm.py --help
```

Version return works as expected with bcbio-nextgen though:

```
$ bcbio_nextgen.py --version
1.1.6a
```

What's the best way to add support for this? I can do a pull request if necessary.