Introduce WebSocket into rTorrent

rTorrent is a stable, high-performance and low resource consumption BitTorrent client. This GSoC project will replace the antique SCGI protocol with Websocket, which will allows real-time events, less serialization/transfer overheads, better security, etc. We will introduce uWebsockets into the rTorrent to make client can communicate with rTorrent via websocket protocol, client can subscribe some specific topics, once the event occurs, server will push the notification to client automatically; implement some callback function and bind to the events we care about, execute them when event occurs. For secuity and convenient reason, we will implement websockets over unix domain socket, which could be access restricted as a file, much efficient and safe than a TCP:IP port. There isn't a modern c++ websockets library that supports unix domain socket, we will add this important feature into uWebsockets, possibly by PR to office repo (if it will be accept) or personal fork.