Added ESP8266 support and returned to PubSubClient for MQTT

The most important changes in this release is that ESP8266 boards are supported now. A few compiler preprocessor directives (#if defined()) are used to add a few yield()-statements for the ESP8266 only. Also, the final StaticJsonDocument is replaced by a DynamicJsonDocument because the ESP8266's stack is not big enough for the static variant.

The Advanced_Thermostat.ino program uses the PubSubClient again as MQTT client. I found a work around for the connection losses by managing the TCP/IP connection form the program and not from the library. This may imply that you have to add the PubSubClient-library using the library manager (search for 'pubsubclient oleary')

The README.md's are extended and a few typo's are removed.

The comments at the head of the examples are adapted.

A few small bugs were solved.

Full Changelog: v1.1.0...v1.2.0

Several minor changes, adding a full functioning Home Assistant Thermostat

Now the Advanced_Thermostat.ino in the example directory is a ready-to-go thermostat specially designed for Home Assistant with MQTT integration. Just set your credentials in the program, flash it to e.g. an ESP32-S2 mini, connect the wires of your existing thermostat to the shield and add a Home Assistant Automation to send the room temperature to the thermostat. The Thermostat (MQTT HVAC) Climate integration will 'automagically' appear, together with sensors for boiler setpoint temperature, router RSSI signal level, a flame ON / OFF sensor, and, if your boiler supports it, water pressure, a flow temperature, return temperature, relative modulation percentage and Domestic Hot Water sensor.

EasyOpenTherm thermostat library 1.0.1

Changes:

• Sync library.properties version with the GitHub version tag
• Minor textual changes

EasyOpenTherm thermostat library 1.0.0

First release of this library that consists of classes to communicate acting like a thermostat (primary) to a boiler or HVAC (Heating Ventilation Air Conditioning, secondary).

Three classes are defined adhering to the OSI model. The top level class, OpenTherm, is used to interact with the boiler. A lof of enum classes are defined with lots of elements to be as complete as possible to cover all different interaction with the secondary, however in a basic program you need only a few, like OpenTherm::READ_WRITE_DATA_ID::CONTROL_SETPOINT_CH.

Several examples and a README.md are provided for a quick start.