Add MQTT protocol codec

MQTT is a open source protocol on top of TCP which is widely used in
mobile communication and also for IoT (Internet of Things) today. This
will add an open source implementation of MQTT so that it becomes easier
for Netty users to implement an MQTT application.

For more information about the MQTT protocol, read this:

http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html
Overall clean-up on codec-mqtt

- Use simple string concatenation instead of String.format()
- Rewrite exception messages so that it follows our style
- Merge MqttCommonUtil and MqttValidationUtil into MqttCodecUtil
- Hide MqttCodecUtil from users
- Rename MqttConnectReturnCode.value to byteValue
- Rename MqttMessageFactory.create*() to new*()
- Rename QoS to MqttQoS
- Make MqttSubAckPayload.grantedQoSLevels immutable and add more useful
  constructor