First pass at removing calls to index.php and replacing with wpsc_ajax.ajaxurl

(Note: this, and other commits, are NOT a refactor.  Lots of standards work still to be done.  Limiting scope to actual ticket issues.)
Updating calls to index.php to be wpsc_ajax.ajaxurl

Functions changed, and therefore, to test:
* Add to cart
* Select variations (price change)
* Shopping cart widget loading
* Empty cart
* Switch shipping method
* Set billing/shipping country
* wpsc_set_profile_country()

Functions not changed, and why
* gallery_list, admin.js:L434 - not found in core.  Probably legacy, possibly GC.  Investigate and remove, possibly.
* shopping_cart_collapser() - I don't believe this actually works :)
* wpsc_upload_switcher() - admin.js :L661 - legacy.  Should be removed, or possibly moved to admin-legacy.js
* fillcategoryform(), admin/js:L751 - same
* jquery.rating.js - this is old, non-i18n - needs to be removed from core and pluginized, reviewed for coding standards and i18n.
* user.js - marked as legacy.  Need to review to see if it's still needed at all.
* product_tagcloud.js - should be removed, no longer enqueued.

FWIW, I've heard the arguments against this (what about .htaccess blocking admin requests?!) - Lame arguments, at least until WP gets a front-end ajax handler.  See http://core.trac.wordpress.org/ticket/12400#comment:20 and further conversation.