Add support for MaxMind Netspeed

```python
>>> self.gi = pygeoip.GeoIP('/path/to/GeoIPNetSpeed.dat')
>>> self.gi.netspeed_by_addr('google.com')
'Corporate'
>>> self.gi.id_by_addr('google.com')
3
>>> self.gi.netspeed_by_addr('defunct.cc')
'Cable'
```