Merge pull request #605 from MPolleke/master

Last NL updates for now.
Change wpsc_get_remaining_quantity() to call the method dynamically, rather than statically.  The previous behaviour caused a strict standards warning.

Fixes #606.  Props @leewillis77
Merge branch 'master' of github.com:wp-e-commerce/WP-e-Commerce
Add minimum of zero to number input fields where appropriate.  Also, a string change for misspelled 'extention', which is not a word.
Reworked wpsc_get_child_object_in_terms to provide better performance when large number of variations and products
Fixing merge conflicts with wpsc_get_child_object_in_terms() optimizations.
Minor whitespace fixes in wpsc_get_child_object_in_terms().
Whitespace / typo
Avoid warning caused by trying to assign the return value of a function by reference
Merge pull request #614 from leewillis77/strict_standards_fix_cat_functions

Strict standards fix cat functions
added static to method decrlarations to avoid php warnings like this

error type: [2048] call_user_func_array() expects parameter 1 to be a valid callback, non-static method Sputnik::add_download_link() should not be called statically file: H:\web-site-dev\sparkle-gear.com\wp-includes\plugin.php line: 173
Merge pull request #624 from JeffPyeBrook/sputnick-static-callbacks

Static method declrations
Allow gateway images to be filtered
Merge pull request #626 from leewillis77/payment_images

Allow gateway images to be filtered