only do article update operations once when scrolling

I discovered that DOM operations and function calls were done several times inside the 100px mirror when scrolling through articles. Especially painful on seamless scrolling systems like Mac OS, where dozens of updates would slow down this part dramatically. This change fixes it.
Merge pull request #236 from levito/patch-5

only do article update operations once when scrolling
add af_dilbert
af_dilbert: fix typo
do not catchup newly subscribed feeds
parser: remove atom <source> element
Merge remote-tracking branch 'upstream/master'