Fetch a Last.fm user's recent listening history and top artists over configurable time periods

Steps

1. Obtain a Last.fm API key (session/auth is not required for these public read methods, just an api_key)
2. Call `user.getRecentTracks` with `user`, `api_key`, and `format=json`, using `limit` (max 200 per page) and `page` to paginate, and `from`/`to` Unix timestamps to bound the range
3. Detect the currently-playing track by checking for the `nowplaying="true"` attribute on the first item, and treat it separately from completed scrobbles
4. Call `user.getTopArtists` with `user`, `api_key`, and a `period` value (overall, 7day, 1month, 3month, 6month, or 12month) to get ranked artists for that window
5. Use `limit` and `page` on getTopArtists as well when a user's history is large enough to need pagination
6. Cache responses appropriately, since repeatedly polling a user's recent tracks at high frequency against the public API can hit rate limits