add some annotations to the methods
1.modified a method name(public void buildHierarchyGraph())
2.modified the matching rules in parseElements()
1.refactor the method parseElements(), and two methods: parseUrl() and parseLabel();
2.add a class Category to record the current web page's label name and url
add some content to Readme.md
1.modified the misspelled words
2.to add the category to database, I use hibernate.