Use system proxies

Detected by checking the environment variables http_proxy, https_proxy, their uppercase variants and then all_proxy. Uses no_proxy environment variable as comma-separated list.

On Windows (and if previous detection fails), the registry is queried for `HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings`, which is what you edit when you open "Internet Options" > "Connections" from the Control Panel or Internet Explorer.

If you want to force a direction connection, you can write:

```php
$conn= new HttpConnection($url);
$conn->setProxy(HttpProxy::NONE);
$conn->get(...);
```

## TODO

* [x] Verify detection on Windows
* [x] Verify detection on Un*x
* [x] Write tests

## See also

* https://wiki.archlinux.org/index.php/Proxy_settings
* https://github.com/xp-framework/rfc/issues/162