Add support for get_notifications

[CloCkWeRX]

https://dev.splitwise.com/#tag/notifications

{
  "notifications": [
    {
      "id": 32514315,
      "type": 0,
      "created_at": "2019-08-24T14:15:22Z",
      "created_by": 2,
      "source": {
        "type": "Expense",
        "id": 865077,
        "url": "string"
      },
      "image_url": "https://s3.amazonaws.com/splitwise/uploads/notifications/v2/0-venmo.png",
      "image_shape": "square",
      "content": "<strong>You</strong> paid <strong>Jon H.</strong>.<br><font color=\\\"#5bc5a7\\\">You paid $23.45</font>"
    }
  ]
}

Enables use cases like: https://github.com/sriramsv/custom_component_splitwise/issues/4