v2.0.0: Merge pull request #59 from 256dpi/v2

• Removed YunMQTTClient (see #57)
• Ported MQTTClient to lwmqtt
• Added clearWill() function
• Added QoS level and retained flag to setWill() (see #49)
• Added QoS level to subscribe() (see #49)
• Added QoS level and retained flag to publish() (see #49)
• Added error and return code getters (see #64)
• Removed publish method with MQTTMessage struct parameter
• Added MQTTClient client(256) constructor to allow simple buffer size override. (see #30)
• Callbacks can be registered using onMessage() (see #55)
• An Advanced callbaback can be registered using onMessageAdvanced() (see #38)
• Replaced all string pointers with array notation.
• Added more subscribe() and publish() variations.
• Pass String objects as reference to functions.
• Add an Arduino Yun SSL example.
• Documentation of messages received during subscribe, publish and unsubscribe.