Doesn't work with a url like mqtt://******

- [x] Bug exists Release Version 1.2.0 ( Master Branch)
- [x] Bug exists in Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Version (mqttv5-new Branch) -- Could not check from github. Sorry!

I tried to connect my server with a URL like a ```mqtt://***``` but was unsuccessful.
It started work properly after using ```tcp://```.

According to following code, ```mqtt://``` raises IllegalArgumentException.
https://github.com/eclipse/paho.mqtt.java/blob/master/org.eclipse.paho.jmeclient/org.eclipse.paho.jmeclient.mqttv3/src/org/eclipse/paho/client/mqttv3/MqttConnectOptions.java#L388

I think specifying ```mqtt://``` is common way to clarify a server is running mqtt protocol.
Could you allow ```mqtt://``` for beginners like me?