Reformat old test cases
Refactor code in products.tests
Add tests for Prodcut.price_at
Add Product.price_at
Reimplement Product.current_price in terms of Product.price_at
Add tests for Product.min_price_at
Add Product.min_price_at
Reimplement Product.min_current_price
Merge pull request #1 from szabba/master

Added price_at and min_price_at; reimplement current_price and min_current_price in their terms