Make port argument optional

Don't force the port argument to be something like '3000'. This prevents you from having to do something weird like this to connect to a published app:

```js
var client = DDPClient({
  host: 'myapp.meteor.com'
});
client.port = '';
```

Now you can just omit the port argument, which looks like this:

```js
var client = DDPClient({
  host: 'myapp.meteor.com'
});
```