Ecomm Add Fixes

I tested out the existing eCommerce integration, but it didn't work with my spree install.

Here are some things I tweaked:

* The taxon locator code is more flexible. It looks through the ancestor tree and picks the first taxon that is at the top of the tree (the one right below your "taxon category", i.e. a `Spree::Taxonomy`)
* Use `price` instead of `cost_price`. `price` is what the customer paid
* Format the `order_date`. If you don't use YYYY-MM-DD mailchimp we just use the current date