Rebuild server/client classes using SocketServer

- server uses ThreadingTCPServer, so clients run in threads as before;
  shutdown also closes the socket, so manual restarts don't block it;
  Sublime 2's python has _BaseServer__serving, while python 2.7 has
  _BaseServer__shutdown_request (with opposite meaning)
- client uses BaseHTTPRequestHandler as previously;
  move code into the intended setup/handle/finish callbacks;
  sending and reading can be altered via on_send and on_read
- remove on_* callbacks, most event methods can be overriden if needed
Moved Webloader files into project root