Rename `MasStoreSearch` as `ITunesSearchAppStoreSearcher`.
Rename `StoreSearchMock` as `MockAppStoreSearcher`.
Rename variables/parameters of the above types as `searcher`.
Fix 'App Store.app' mention in help.
Partial #585
Signed-off-by: Ross Goldberg <484615+rgoldberg@users.noreply.github.com>