Hook wpsc_ajax literal to wp-e-commerce script instead of wp-e-commerce-dynamic - as I'd like to look into deprecating that.
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
Add minimum of zero to number input fields where appropriate.  Also, a string change for misspelled 'extention', which is not a word.