Itskins' new Zero.3 for iPhone 5 ($18) is a step above the standard, generic plastic shells we've seen from so many companies. Offered in six colors, it's a soft, semi-flexible plastic, rather than the rigid material shells are usually made of, and quite thin at only 0.3mm per side. The case snaps onto the back of your iPhone, with openings for each of the buttons along the top and side, plus an open bottom edge. Its design matches that of the iPhone…