Context manager support

It would be more Pythonic if you guys support Python's context manager for `geoip2.database.Reader`, so that we can call:

``` python
with geoip2.database.Reader('GeoLite2-City.mmdb') as reader:
    r = reader.city(ip_addr)
    data = {
        'country_code': r.country.iso_code,
        'country_name': r.country.name
    }
```

and `reader.close()` is called automatically when exiting `with`.
