Support for scheduled pushes

[Aziz38]

We have easily setup OneSignal push service with this adapter because of hard work you have done here. Thanks you very much for that but it would be really superb if you could add support for scheduled pushes too as this is essential for our app. Now no matter what we set to "push_time" parameter on Parse.Push.send(), pushes are sent immediately.

[yingliangzhang]

There is a pending PR for this: #22

However, I have made a forked version which you can use if you are keen
https://www.npmjs.com/package/parse-server-onesignal-push-adapter-scheduled-push-version

[juliusbtesh]

I was never able to get this working. It showed that something was set up and scheduled but all the notifications went out right away