Push notification support added for Android devices

- Data models updated for using push notifications. Added fields to the User schema:
  devicePlatform, pushNotificationId

- Push notification id also can be updated later(The id may be received by device after the connection establishment to the server.)

- Now, push notifications are sent to not online receivers by the server(if they have a push notification id saved in the server), when they receive a recommendation request or reply from other people.

@TODO: some more testing on the server side, Apple Push Notification Service registration (needs keychain, certificate files,...)