When user1 sends a message to user2, user2 will receive a notification with format: iOS
{
  APNS: {
    aps: {
      alert: {
        body: first 40 characters in message's content. If message is
        file/location message, we'll send "You have new file/ location"
      },
      sound: "default"
    }
  },
  APNS_SANDBOX: Same content with APNS,
  category: "conversation",
  id: conversation_id,
}
Android:
{
  GCM: {
    notification: {
      text: first 40 characters in message's content. If message is
        file/location message, we'll send "You have new file/ location"
    }
  },
  category: "conversation",
  id: conversation_id,
}
iOS
{
  APNS: {
    aps: {
      alert: {
        body: Notification's content
      },
      sound: "default"
    }
  },
  APNS_SANDBOX: Same content with APNS,
  category: "reservation",
  id: reservation_id,
}
Android:
{
  GCM: {
    notification: {
      text: Notification body
    }
  },
  category: "reservation",
  id: reservation_id,
}