Shopkit v0.9.1

Part bugfixes, part code optimization, and part new features! There are still a couple major features I'd like to include before v1.0, but it didn't make sense to keep you waiting. So here's v0.9.1!

• Cleaner object-oriented code (thanks to contributions from CelineDesign)
• Featured products (one-click purchasing from the sidebar)
• Product thumbnails in the cart
• More versatile product page layout with simpler "Buy" buttons
• Product options for variations that don't affect an item's price (e.g. t-shirt colour)
• Pre-filled default values during shop setup
• Visual tweaks
• Bug fixes