More blacklisted selectors
Minor comment changes
Refactored polling

lucu.pollFeeds now uses caolan/async.js internally. The pollActive check
is no longer in use. The idle check that was present in background.js
was moved into poll.js. pollFeeds now pre-builds a feeds array using a
single transaction which avoids opening a db connection for each feed
after fetch. Dropped support for callback that passes entries processed
and entries added metrics.