Visitor Meta Fixes to initialization sequencing 

* Test for presence of wpsc_action in addition to testing DOING_AJAX
* Initialization of a return value was missing
* Check for wpsc_action in addition to checking for DOING_AJAX
* Fixed incorrect wpdb property tables names for cart item meta and purchase meta in upgrade routine
* Check for improperly stored meta structured when retrieving cart add some utility hooks
* AJAX url need to be absolute so that pages served from CDNs send AJAX requests to the correct host.
* Add wpsc internal filter to customer validation response. Useful for future extensibility, and also useful for debugging and testing.
* WPEC Visitor Validation Javascript Improvements making customer validation code to more reliable.
* Added logic to prevent more than one validation attempt per session, avoids slowing down pages when WPEC server is not responding promptly.
* Check data integrity for cart meta values retrieved from WordPress meta
* Use get visitor cart function inside of visitor class rather than replicating the code.
* Don't store empty cart to visitor meta on conversion if it can't be deserialized from WordPress user meta
* Fixed and test customer init javascript, removed set of timeout since that is not permitted on synchronous requests.
* Fix database initialization of visitors table on new install

*Fix for spurious emptying cart*
* Added check for update_product_price as an ajax action, it appears to be the only action that still does not contain 'wpsc'.
* Changed ajax action test to contain rather than starts with.
* If there is a valid customer id in a cookie we use that rather than test for a robot