The long, slow rollout of Google Glass continues. Back in May, Google started selling its $ 1,500 Glass “Explorer Addition” to anyone who wanted it after months of exclusive, invite-only availability. Now, Google’s made it an official part of its biggest storefront, the Google Play store. Of course, it’s still the expensive Explorer Edition, rather than some finalized (and less pricy) consumer product — but the more places Google sells Glass, the harder it is to make that distinction. Most people who stumble upon Glass in the Play store won’t necessarily understand that “Explorer Edition” is just another way of saying “beta,” but developers who want to start building good Glass apps should still get a lot of milage out of the headset….