C++ implementation of an MQTT-SN client and gateway

Minimal MQTT-SN client and aggregating gateway, with gateway discovery supported
Intended for the Arduino environment, client is light enough to run on an Uno easily
Completely non-blocking, so other application tasks can happen during protocol transactions
Modular, so that new transports and HALs/devices can be easily added by implementing the right interface
Gateway falls back to being a local MQTT-SN broker, in the absence of an MQTT connection
Zero dynamic allocation, up-front costs only, a plus depending on your application
Basic functionality complete and tested. No topic wildcards, LWT or message retention supported yet
Sleeping clients now supported, though completely untested for now

Dependencies

A modified fork of [knolleary]'s PubSubClient provides the default MQTT client interface in examples. This can be replaced by simply sub-classing the MQTTClient class to provide your own MQTT client implementation.
A simple generic FIFO library for holding messages