Support for slow client

[sergei-grigorev]

I've tried to find a way to use netty callbacks and checks if netty buffer is ready to consume more information. Please review this small patch to #172

Example, how it could be used (where client: SocketIOClient, and ctx is context of netty ctx: ChannelHandlerContext)

if (client.isWritable) client.sendEvent("server_response", msg)
else {
  ctx.channel().config().setAutoRead(false)
  client
    .sendEvent("server_response", msg)
    .addCallback(r => if (r.isDone) ctx.channel().config().setAutoRead(true))
}

In deep it uses netty futures and promises but outside only pure java.util.concurrent.Future and some new interfaces in the package com.corundumstudio.socketio.concurrent in order to make it possible to use another implementation.