better pixel-aligned 16px icon

Previously, the 16px icon was the vector icon (mastered at 48px) scaled down to
16px. It needed a bit more love. It was tricky to get right; ultimately used a
6px diameter lower left circle, and a 6.5px diameter upper right circle.

7px diameter circles filled the icon nicely, but such an arrangement could not
accomodate a good pixel-aligned arrow (good is when the connection between the
circle and arrow bisects the arrow's right angle). 6px for both circles yielded a
figure that was too small and cramped.

Also optimized the PNGs for size.
pull ping() out of pingTest()

ping didn't benefit from being closed within pingTest, so pulled it out for
increased clarity.
fix incorrect closing tags in popup.html
pull looping behavior of pingTest() out of transition()

the functionality of transition() was overloaded; it should have been only
responsible for updating the UI, but it was also the component that sent out
the next ping (and was therefore responsible for the continuous/looping nature
of pingTest()). Now, the continuous aspect of pingTest happens more explicitly
in a function called testContinuously.