Fixed variable error in wpsc-shipping/usps_20.php

The variable being tested is NOT the one being assigned the new instance of ASHTools().
Filter gateway checkout form output
Add docblocks for all functions in gateways.php
Fix instances of "siteurl" that should use home_url().
Remove unused  $siteurl references.
Use site_url() instead of get_option( 'siteurl' ).
Use admin_url() instead of "siteurl" for admin URLs.
Erroneous variable $draft, should be 'draft'.  Props benhuson
home_url() should include slash when adding query args. Props meloniq
Use get_edit_post_link() for product edit link. Props meloniq
Eliminate Undefined variable errors in wpsc_obtain_the_description

This poorly-written function is now less poorly-written. Removed
unneeded globals, test $_GET variables before using, removed direct
query to database
Typo in last commit

Got too carried away
Stop testing for category ID - just return output if set

Poor logic begets poor logic
Fixes issue 302.  Props eangulus
Fix selected state for purchase status links in Sales Log list.
Adding four new filters on purchase log SQL Query

Primary use case in my head here is for plugins using wpsc_sales_log_process_bulk_action and wpsc_sales_log_process_extra_tablenav - they should be able to easily filter the actual SQL query, not the items returned.
move payment gateway settings inline, removing the settings sidebar.
render list of payment gateways more similar to the wordpress plugins page.
improve handling of display names for payment gateways
add current display name to payment gateway table. improve cancel button on payment gateway settings rows.