Rework dividends saving model

This issue will have to wait after #4

Right now we are saving dividends info into a json file, which we might not be able to connect with our services. When a user decides to say that he had received a dividend, how can we connect the json and the user's money amount. We will need to save how much stocks the user had at the ex-date and how much he had received. That will be tough. And that is the reason we won't prioritize this task.