Pull request for variation improvements - issue #153

Pull request for variation improvements - issue #153

Updates to cart class and related functions to support product options that can change prices, and do not rely on "hijacking" the custom_message field.

highlights:
1) item_meta protected field added to cart_item class,
2) functions added to get/set/delete cart item meta values.
3) actions added to allow theme and plugin delopers to display cart item
custom values in a manner similiar to how custom_message is handled

Using a filter just as is done in wpsc_cart_item_url() would be consistent. Issue #148