[mqtt] Working proof-of-concept implementation -

Currently it's very simple and can only publish on QoS level 0.
There only one function and it can be optionally compiled with
XI_MQTT_ENABLED=1 passed to make command.
[mqtt][example] Add first simple PoC example
Merge pull request #102 from xively/feat-mqtt-codec

[mqtt] Working proof-of-concept implementation