Changes

tsk14 Creates method to explore the stories

tsk14 Updates tests for exploreStories method
Fixed the queries for the published date in method exploreStories 
Created more tests to test differents searches by genre, topic and
published dates

tsk14 Modifies tests for exploreStories method
Created different methods in StoryDataset to get the number of stories
by genre, topic and publication. These methods allow us to get the
stories from the dataset dinamically instead of hardcoding this number.