Improve the MQTT interface: Use persistant connection, publish the timestamp, use the Homie library/specification

[Drolla]

There are 3 improvements regarding the MQTT interface that have been originally proposed by @rkoshak:

• The use of a persistent connection in combination with an LWT message
• Publishing of the timestamp
• Use the Homie library/specification

[Drolla]

The MQTT module has been rewritten to use a persistent connection and a LWT message. Sensor data can be published together with the sensor read timestamp. In addition, a "publish timestamp" is published together with a status (Online, Offline, Connection lost). So, this addresses the 2 first points suggested by @rkoshak. For the 3rd suggested point, a separate ticket has been opened (#14 ).

[Drolla]

Published timestamps: