Addition of Shipping Services, Economy, Plus & Premium.

 As per http://www.shipwire.com/w/developers/xml-shipping-rate-api-faq/
Added a check to make sure post object is set before trying to access it
Update whitespace & braces to WP Coding Standards

Just a bit of nit-picking related to WP coding standards. No
functionality has changed.
Use get_the_terms() instead of wp_get_object_terms() when duplicating products.

Issue #100.
Replace almost all wp_get_object_terms() with wpsc_get_product_terms().

The only exceptions are calls to wp_get_object_terms() with multiple $ids as
arguments.

Issue #100.
Fix fatal error on front end.
Get rid of global $current_screen.

Issue #31.
Use correctly spelled constant WPSC_TABLE_SUBMITTED_FORM_DATA.
dropdown options is a string, not an array.
- working through docs
ignoring PHPstorm files
more docs
- finished docs on the file
- lining up with WordPress coding standards

Switched to tabs for beginning of line and spaces mid line which is what
the WordPress coding standards want.
working through documenting the admin.php file
cleaned up formatting more so it all matches
removed terminating ?> since it is not needed
making it further in the docs for admin.php
finished docs on admin.php file
updating with changes from garyc40 notes