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 product view images (i.e. front, back, side, etc.)
- [x] Customize the "Product Details" metabox
- [ ] Customize "Product Options" (i.e. size, color, etc.)
- [ ] PayPal integration
- [ ] ~~Product Reviews (uses built-in comments)~~
- [x] __Function__: `steel_product_meta()` to get metadata for products
- [ ] __Function__: `steel_product_views()` to display product view images
- [ ] __Flint Template__: Display a single product with description, details, and product view images
- [ ] __Flint Template__: Display product category with category description and product gallery