Add support for silent notifications

[Amareis]

In Amareis@5f78eea I made support for silent notifications, using iOS 9 methods. Can we implement it in this fork? It's very useful feature.

[Amareis]

@chrisjpalmer @CowboyCode hey, i see you are active contributors. Can you say something about subj?

[luigi37]

Hi Amareis. What is the difference versus suppressing sound option in the payload?

[blumanski]

We dropped ios9, silent notifications should work already

[Amareis]

@luigi37 silent notifications is not notifications, what not hears by users. It's when ios awake your app and give it chance to download some info updates from server, just Google "content-available ios"

[luigi37]

I see so silent both in sound and visualisation, right? Does your code work for both android and Apple?

[Amareis]

@luigi37 when your app sleep in background and notification is arrived, you may want to download some info from server. In android there is no problem, because android always awake your app when notification given, but ios will awake app only if you set "content-available" flag in notification payload

[luigi37]

I see. Looks interesting

[chrisjpalmer]

Thanks. I will check out your fork and try and implement it. Time frame: 2 weeks, unfortunately I have a paid full time job.

Thanks

Chris