Update Gateway_Ethernet.ino

Removed most MQTT connection watchdog code in favor of a method I found at https://github.com/knolleary/pubsubclient/issues/41 . I haven't tested it extensively yet but it seems to be simpler and hopefully more stable.

Commented the heck out of everything

Also added a functional MQTT subscription ability. I think it's a good first step towards making this setup bi-directional