Migrate from GCM to FCM

Foreground listener

fix background PN is received as foreground

for the first time. refs #268 
The `didReceiveRemoteNotification` checks for the current app state which will be active and hence wrongly emit the notification to JS as foreground. However at the exact time when the user opened the notification, the app was at the background hence this is a background notification by definition.

In other words, conceptually determining if the notification is background or foreground should NOT be based on the time it was consumed from the queue but on the time it was received. Practically, if it was queued, then the app must have been in the background ==> background notification.

Update gradle files, nothing fancy.

[FIX] Android 8

Update notificationsEvents.md

Added method for getting the current iOS badge count

getInitialNotification implemented on iOS

iOS opening Local Notification crash fix

Changes the documentation so the example is correct in the prevention of the memory leaks.

`.bind` will return a new version of the function on each call which means `removeEventListener` won't be able to find the previous listener and release it.

This reverts commit 9a47c853.

Fix setting of the `openedLocalNotification` to use the `userInfo` from the `UILocalNotification` if it exists instead of the `UILocalNotification` itself

Break README into sub-sections in separate files with a ToC

Remove @providesModule