accept different ports for multiple hosts

At the moment, the module requires all the hosts to have amqp server running on the same port. However. this might not be the case.

This change allows to specify different port for each server. E.g. 
```javascript
var connection = amqp.createConnection({host: ['www.server1.com', 'www.server2.com:5673']});
```

The default server port will still be used if no port is specified in the host URLs.