fix(one-signal): return Observable instead of promise (#352)
fix(one-signal): return Observable instead of promise (#352)

* Added OneSignal wrapper

* documentation

* Changes for callback of notification revieced

* fixes for @Cordova decorators without Promise return

* Merge

* Improvements to OneSignal
extended init function with notificationOpenedCallback as an optional parameter

* Platforms removed
OneSignal supports more than only these 3 platforms. It's available to nearly all Cordova platforms.

* Init method turned into observable

ramonornela/ionic-native,driftyco/ionic-native,xueron/ionic-native,xueron/ionic-native,driftyco/ionic-native,xueron/ionic-native,driftyco/ionic-native,xueron/ionic-native,ramonornela/ionic-native,klirix/ionic-native,klirix/ionic-native,klirix/ionic-native,ramonornela/ionic-native,ramonornela/ionic-native