Returning "this" for command methods so we can chain method calls.

Minor change, but it lets us write code like:
```js
var bot = new Steam.SteamClient()
    .logOn('username', 'password')
    .on('loggedOn', function() { /* ... */});
```

instead of 

```js
var bot = new Steam.SteamClient();
bot.logOn('username', 'password');
bot.on('loggedOn', function() { /* ... */});
```