add CategoryDaoImpl
add getCategories method
add getCategory method
add insertCategory method
add updateCategory method
add deleteCategory method
extract interface CategoryDao
add test CategoryServiceImplTest
add test should_return_categoryList method
extract interface CategoryService
modify getCategories method
add test should_return_category_when_input_id
modify getCategory
add test should_insert_item
modify insertCategory
add test should_update_item
modify updateCategory
add test should_delete_item
modify deleteCategory