Public customer meta functions to separate file

Moved customer meta functions that can be called out of the file that defines the WPEC customer
internal processing.  Named the file so that it appears with the other meta related files that exist
as of this commit, wpsc-meta-cart-item, wpsc-meta-purchase, wpsc-meta-init.

The new wpsc-meta-customer file more less defines the public customer meta API.

Resolves issue #841