export the Socks5ClientSocket constructor and createConnection() as a st...

this should close https://github.com/mattcg/socks5-client/issues/3

also, wanted to expose the `Socks5ClientSocket` constructor, since not all use cases will involve immediately opening the connection using `createConnection()`

now you should be able to:

```js
var Socks5Client = require('socks5-client');
// create instance
var socket = new Socks5Client('localhost', 9050);
// or create connection
var connection = Socks5Client.createConnection({
  socksHost: 'localhost',
  socksPort: 9050,
  host: someHost,
  port: somePort
});
```