Steel 1.2 - Marketplace

Steel 1.2 ships with our latest module, Marketplace, which allows you to easily create an ecommerce part of your WordPress website. Marketplace will replace Sparks Store, and we will have a guide to switching soon.

Marketplace features:
- [x] Add/edit products
- [x] Add/edit product categories
- [x] Add/edit tags to products (uses built-in `post_tag`)
- [x] Add/edit
- [x] Add/edit product view images (i.e. front, back, side, etc.)
- [x] Easy function `steel_product_meta()` to get metadata for products
- [ ] Easy function `steel_product_views()` to display product view images
- [ ] Customize the "Product Details" metabox
- [ ] PayPal integration