shipment_serializer done. Closes #7

adding some more specs. wip

implement the shipment serializer specs as well
Introduce payment serializer
Define push_it inside serializers

Make it easier to stores to hook the after commit and not polute the
Spree::Order, for example, interface. So instead of class eval we could
do something like:

    Spree::Order.after_commit -> { Spree::Hub::OrderSerializer.push_it self }