Localization of WPeC user javascript variables is now more explicit

The loop that was taking the localized admin js vars out of the localization structure and putting them into the window content has been replaced by explicit assignments. Although it is a little more code than the prior loop it should now be easier to see what is happening.
For localized js vars added after 3.8.14 three utility functions have been provided to access localized vars
 wpsc_var_get ( name )
 wpsc_var_set ( name, value )
 wpsc_var_isset ( name, value )

This PR also contains the user side of the audit required to close issue #1110