Calculate shipment total for an order based on shipment's cost + promo_total

A shipment with cost = 5 and promo_total = -5 will have a final cost of 0
[frontend] Display just the shipping total in checkout/_summary, rather than each shipment
Correct negative amount checking in frontend specs

Due to recent changes in Spree::Money, which changes position of negative symbol
[frontend] Correct subtotal check in page_promotions_spec
Correct indentaiton of jQuery.payment code in checkout/payment code
[frontend] Correct shipping total check in checkout_spec
[frontend] use correct class to check cart total in coupon code spec