1- Refactor multiple choice question types directives. This includes:
    1.1- Add 'ng-class' to letter button, when the 'selected' property change it affects directly to add or remove the required class.
    1.2- Finish to remove/reduce jQuery selectors. In order to avoid too many access to the DOM.
    1.3- Add 'ng-click' method to each row on each directive. This is how it executes now the logic to select a choice.

2- Remove old services dedicated to handle multiple choice directives.
3- Remove old jQuery 'existsInArray' service.(The logic related to this service is now using lodash).
4- Remove un unused 'easyPieChart' directive.(This wont be needed since we removed the old way to show the score prediction).
5- Add missing call to 'loadQuestionSet()'.

*(2)- Since we work directly with scope value, maybe will more readable keep the directive code inside the link function instead of have it into a service(this can be evaluate it later).