removed unreferenced global.
removed call to get_post_type_object() whose return value was never used.
query will retrieve post types of 'wpsc-product' by default, consistant with other similiar queries.
query post type is overridable using a new filter that is similiar to the other filters on the same query.
fixed whitespace
restored erroniouslly removed global variable
Merge pull request #829 from JeffPyeBrook/query-start

Start product query fixes