Change product_url to product_path.

I'm not sure why product_url was introduced, but I'm currently
encountering issues in production where it's not respecting subdomains,
HTTPS, and ports correctly.

Nearly everything else in Spree links to stuff using the _path helpers,
which I like a lot better.
Move price in to an offers block.

This is what the correct schema.org product implementation is.

Reference: http://schema.org/Offer
Merge pull request #4 from gmacdougall/2-3-dev-url-never-path-forever

Change product_url to product_path.