Custom Object Meta Support

Implements custom object meta support using the WordPress common meta
infrastructure.  Includes access functions for known WPEC object types.
Access functions mirror the WordPress 'post' meta functions.  Includes
conversion routine to move data from wpsc_meta to new architecture.
Includes functionality to allow developers to generate meta support for
new custom objects.

As discussed in #400
Changes based on review feedback

Fixed spaces, removed debug cruft, removed tick marks from SQL to be
sent to dbDelta
Merge branch 'Custom-Object-Meta' into branch-3.8.11.1