fix for Node v8 (Object.setPrototypeOf called on null or undefined)

[DoNck]

I am a user of Node-RED I was using the output node "node-red-artnet-node" relying on this "artnet-node" package and it came to prevent my Node-RED from starting after upgrading to Node-RED v0.17.5 and Node.js v8.9.1 because of TypeError: Object.setPrototypeOf called on null or undefined

To fix it, I refactored the server stuff following the advices found at: https://stackoverflow.com/questions/39099527/typeerror-object-setprototypeof-called-on-null-or-undefined

I'm not a expert of javascript nor of Node.js and couldn't try my new version with Artnet hardware. I think it should work again but PLEASE READ CAREFULLY MY PR BEFORE MERGING ! At least, my server doesnt crash anymore at startup nor when trying to send Artnet stuffs from Node-RED :D