Find apps in a store by keyword

For finding apps and their store IDs, not for checking ranks. On iOS the list follows the App Store's own search order when Sonar can read it, with the iTunes Search API's relevance order as the fallback (which can sit dozens of places away from the App Store's real ranking below the top 10), and the response doesn't say which one served it, so the list order is not a rank. For an app's position on a keyword, track the keyword and read /api/v1/apps/{id}/rankings or /api/v1/keywords/{id}/rankings.