Android O notification channels

[iernie]

I couldn't find an issue about this, so I am making one.

Given that Android O is just around the corner, the library will need to be updated to use notification channels in order for it to work on the next version of Android, and I guess a way of configuring the name of that channel. If not, no push notifications will show up on Android O and up.

NotificationCompat.Builder(Context context) has even been deprecated in favor for NotificationCompat.Builder(Context context, String channelId)

Read more about notification channels here:
https://developer.android.com/preview/features/notification-channels.html