Extend Connection Model to attempt re-connect based upon user-supplied reconnect interval

[bblacey]

Currently the MQTT client is a publish only client however the underpinnings for a persistent connection model are already in place. Currently, the client will not reconnect to the broker unless it has something to publish. To improve future support for subscription support, the connection model should be extended to reconnect after a user-specified interval elapses. This is a very straight-forward change - check if the reconnect interval has elapsed in the processMQTTevents() method and if so, attempt to reconnect to the broker.