[TIMODOPEN-270] Adding PushNotification methods

##### Steps to test:
1. Run the example app.
2. Login with an admin account.
3. Go to PushNotifications > Settings and enable push notifications.
4. Go to PushNotifications > Subscribe and subscribe to a channel, name it what you want ("test").
5. Go to PushNotifications > Show Channels enter the channel you just subscribed to ("test") and click Show Channels. You should see the device counts for that channel.
6. Go to PushNotifications > Query Notifications and you should see the channel that you subscribed to ("test").