wpsc_get_remaining_quantity was using the global cart class, intialized or not, to get the remaining quantity of a product
cart::get_remaining product quantity was not using any class functionality.

Put hte get remaining quantity logic in the standalone function, have the class function call the stnadalone
function for backwards compatibility.
Cart Qauantity Error on Uninitialized Cart
Moved remaining product quantity logic from class to the existing standalone function. Class function now calls

standalone function rather than the other way around.

Global cart was being accessed to in the standalone function even though it may not have been initialized.
Detect AJAX processing for legacy cart operations that to not include wpsc in the action name
Merge branch 'pr-visitor-meta' into dev-master
Merge branch 'pr-remaining-quantity' into dev-master