first pass WooCommerce 3.0 compatibility.

[helgatheviking]

Notes:

1. back-compatibility with pre-3.0 is not preserved. To preserve it you would need to write a helper function that always checks the WooCommerce version number and returns $product->id or $product->get_id() as appropriate.
2. product data is not saved via CRUD. But get_post_meta() and update_post_meta() are still viable.

[madebycaliper]

I just spent the last hour going through this on my local install so I could quiet down the PHP warnings... @greatwitenorth it would be awesome if you could merge this PR into master! (I think there's another one waiting on the list that has some WC 3.0+ fixes too, though they might be redundant)