bugfix: typo
impl get_top_selling_free_app_in_category
add red test
impl get_top_selling_paid_app_in_category
add red test
impl get_overall_top_selling_free_app
add red test
impl get_overall_top_selling_paid_app
add test: get_developer_app_list
refactor: indent
add debug flag
refactor: remove unused param
add tests
refactor
refactor: exract to method