Upgrade guide from v1 to v2

This is an upgrade guide to upgrade your Homie devices from v1 to v2.

New convention

The Homie convention has been revised to v2 to be more extensible and introspectable. Be sure to check it out.

API changes in the sketch

1. Homie.setFirmware(name, version) must be replaced by Homie_setFirmware(name, version)
2. Homie.setBrand(brand) must be replaced by Homie_setBrand(brand)
3. Homie.registerNode() must be removed, nodes are now automagically registered
4. If you've enabled Serial logging, Serial.begin() must be called explicitely in your sketch
5. Remove the HOMIE_OTA_MODE in your event handler, if you have one
6. The Homie.setNodeProperty() signature changed completely. If you had Homie.setNodeProperty(node, "property", "value", true), the new equivalent syntax is Homie.setNodeProperty(node, "property").setRetained(true).send("value"). Note the setRetained() is not even required as messages are retained by default.
7. Recheck custom event handlers and direct MQTT topic assumptions against the v2 convention before deploying; the public MQTT layout changed between Homie convention v1 and v2.