[WIP] add ping method and auto-ping-pong

[cwolters]

Add a ping method and an auto-ping-pong operation to detect interrupted connections and allow auto-reconnect afterwads.

---

Added a simple ping operation to allow an easy ping/pong.

ws.on('pong', message => {
// message -> {
//    "event":"pong",
//    "ts": 1511545528111,
//    "cid": 123
// }
})
ws.ping(123)

In case of no channel action (e.g. a single auth user where no messages are flying around), the ping/pong messages will keep the lib from reconnecting due to watchdog timeout..

Currently there is no auto-ping-pong build into the lib. You need to build it on top.
Would be a nice extension to the existing watchdog - so that it triggeres a reconnect only if no ping/pong comes through.

[f3rno]

Thanks for this; could you also add an autoPing option to the WSv2 constructor that sends pings out on an interval when the connection is open? Ideally the interval would be specified with pingInterval or so, maybe defaulting to 1s.

If not I can do it, and would merge this in before, so let me know. Great idea overall.

[cwolters]

Hi Cris, I can do it, but it will take a few days before I can work on the topic. So let's keep this pull request and I will update it later.