Added Product Resource Counts
Added Product Resource Counts

Added resource counting & count resource validation

 * generic validProductResourceCount( $resource )
 * generic getProductResourceCount( $resource )

specific functions for
 * images
 * videos
 * skus
 * rules
 * custom fields
 * configurable fields
lord2800/bigcommerce-api-php,Lewiscowles1986/bigcommerce-api-php,bigcommerce/bigcommerce-api-php,bc-jz/bigcommerce-api-php