introduce hierarchical class-extraction method.
actually use entities for categorify.
fixing minor error in the related ontologies
showing numbers on the categories search tree
Merge branch 'staging' into 476-number_of_ontologies_in_category_on_categories_index
Merge pull request #544 from ontohub/476-number_of_ontologies_in_category_on_categories_index

Number of ontologies in category on categories index.