v0.7.3 - MQTT motion sensor & cleanup

[smeisner]

Changes:

• Add motion device to MQTT discovery
• Move state machine logic into its own function
• Improved the state machine logic to set/unset all GPIO pins on each state change
• Cleaned up some of the mqtt.cpp module
• Incorporated some of Michael B's feedback to improve string matching

Small changes:

• Bump version to 0.7.3
• Removed some dead code in mqtt.cpp
• Stop printing the MQTT user password

[smeisner]

After running new code for almost 2 weeks on an active thermostat, the code is running reliably. Therefore, committing the PR