Refactor FeedFetcher.

FeedFetcher should just download and parse the feed, leaving the rest,
like updating the database and fetching favicons,
to FeedUpdateServiceInternal.updateNextFeed(). This makes the flow
clearer and more flexible, paving the way for fetching feeds that
aren't in the database yet.
Move XMLHttpRequest callbacks from closures to separate functions.
Make subscribing to feeds easier by setting Brief as content handler.

This involves creating a protocol handler for a custom brief:// scheme.

Fixes issue #15.
Merge pull request #51 from ancestorak/improved-subscription

Simplify feed subscription.