make timestamp updates smarter and more efficient

When the timestamp <div>s are created, the “relative-time” class is
added to relative time <div>s that need updates. They are updated
periodically using $(".relative-time").each(...). The “relative-time”
class is removed when the timestamp become absolute and no need to
updates anymore.
Merge pull request #16 from mg6tqq/gap-noscroll

make timestamp updates smarter and more efficient