Refactor to include limit parameter to reduce bandwidth\n\nRefactor calls to chart.getTopArtists, chart.getTopTracks, tag.getTopTags and user.getTopTags to include the limit parameter (where available) to reduce the size of data sent by Last.fm.\n\nFor example, getting limit=1 can reduce receiving 101 items to 1, making the test take 0.5s rather than 1.2s.\n\nAlso return a list of TopItems rather than just items, and add cacheable parameter.