1.6.0

• [fix] Make ETag header comply with standard. (#2603)
• [feature] Loading client script on demand. (#2567)
• [test] Fix leaking clientSocket (#2721)
• [feature] Add support for all event emitter methods (#2601)
• [chore] Update year to 2016 (#2456)
• [feature] Add support for socket middleware (#2306)
• [feature] add support for Server#close(callback) (#2748)
• [fix] Don't drop query variables on handshake (#2745)
• [example] Add disconnection/reconnection logs to the chat example (#2675)
• [perf] Minor code optimizations (#2219)
• [chore] Bump debug to version 2.3.3 (#2754)
• [chore] Bump engine.io to version 1.8.0 (#2755)
• [chore] Bump socket.io-adapter to version 0.5.0 (#2756)