Product with variations lowest price should include out of stock variations

Fix for issue #211

Introduces a wpsc_product_variation_price_from() function which allows extensible passing of arguments to customise the output eg including variations that are out of stock, or not.

The existing wpsc_product_variation_price_available() function now just uses the above function instead and can be deprecated.

Default behaviour of lowest product price with variations is now to show the lowest variation price including variations that are out of stock.