Fix cache bug where torrent info always fetched

The top() function was passing a None default value to the torrent
cache, when it should be using the Scraper object's default value if
None is supplied.
Cut off the top list earlier to fix prefetch

When prefetch was set to true in the top() function, **every** link on
the page got prefetched, which is overkill if you only want 10 links.
Take only the top 10 links earlier in the function, THEN process them --
that way, we only prefetch the links that are requested.
Add some basic docstring documentation