Fix the Node.js run broken in Windows.

Hi, my previous pull req #39 seems has broken Node.js run in Windows,
because some necessary variables were absent in the added env parameter. ( as mentioned [here](http://docs.python.org/2/library/subprocess.html) )

```python
 proc = subprocess.Popen(self.command,
        stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
        shell=shell, universal_newlines=True, env=self.env)
```

So I fixed it by making a copy of current sys env first, as:
```python
self.env = os.environ.copy()
self.env.update(env)
```