Add the 'bind_address' configuration key.

Hello and thank you for statsite!

This pull requests adds really basic support for setting the daemon's bind address. Tests included. Also added an extra INFO log line:

```
statsite[4309]: Starting statsite.
statsite[4309]: Listening on tcp '0.0.0.0:8125' udp '0.0.0.0:8125'.
```

That's about it.