Adds command to list currently installed virtual machines

```bash
$ iectrl
  Commands:
    # ...
    nuke [names] remove all traces of virtual machines
    list         list available virtual machines
    open [options] [names] [url] open a URL in IE
    # ...

$ iectrl list
Available virtual machines:

IE8 - Win7
IE9 - Win7
IE10 - Win7
IE11 - Win7
```

I personally find this useful, since I switch between machines that may or may not have the same set of IEVMS installed. Please critique/reject as needed :)