adds silent push notification support for GCM

[valeryq]

In continuation #42!
Changes compatible with this library https://github.com/zo0r/react-native-push-notification.
Not tested on native Android push notification libraries.
Can someone have ideas how to do it better?

@alex-friedl @appfeel

[coveralls]

Coverage remained the same at 100.0% when pulling 526ee5c on valeryq:master into 94306de on appfeel:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 1e8be59 on valeryq:master into 94306de on appfeel:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 3fd200e on valeryq:master into 8035461 on appfeel:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 745518e on valeryq:master into 8035461 on appfeel:master.

[noambonnie]

Is there anything I can do to help get this pull request merged? I could really use this functionality...

[alex-friedl]

Hello @valeryq,
thank your for your support!
I would prefer a solution without a new config flag, because it comes to down to the library not setting any unwanted fields and the user knowing which payload to use for silent push notifications.

I created a new branch and opened #83.

I don't have a working iOS setup for testing the feature, but could you maybe do so and give me feedback if this solution works for you?

Thank you! :)

[valeryq]

@alex-friedl hi, ohh, I'm already forgot details. But yes, I think I can help in near future, because I also need a web push notification support in this library.

[alex-friedl]

Closed in favor of #83. Please let me know if everything works as expected and file an issue if something comes up :)