Fixed unrouted airwire on VBUS net

Somehow I ripped up a track after exporting gerbers, and committed the wrong
psu.brd. This commit fixes that. The gerbers looked right, and I think they
were, but I regenerated them all to be sure - only the top copper layer was
minorly affected by the fix. Eagle layouts are XML-based, so you can see the
fix in the diff.

Also switched from pdfs to pngs for all exported output. Makes it easier to
view in-browser. One image for the schematic (was too small in last commit),
one for eagle board layout, and one for gerbers from gerbv.