This adds a flag to ProxyRequest to disable the setting of x-forwarded-[for|port|proto]
(will change default to true in next commit)

closes issue #72

[api doc test] node-http-proxy now emits `websocket:*` on important WebSocket events. Added tests for these features and updated some code docs

[fix] Change sec-websocket-location header when proxying WSS --> WS. Added test coverage for this scenario

[fix] Set `x-forwarded-for` from req.connection.socket.remoteAddress if req.connection.remoteAddress is not defined