Usage on windows

Hi, thank you for an awesome project. I have an issue running it on windows though.

Here is the command, I'm running:
```cmd
python multicast-relay.py --interfaces 10.8.0.4/24 "Ethernet" --relay 224.0.2.60:4445
```

And the error I'm getting:

```
AttributeError: module 'os' has no attribute 'fork'
```