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 }

@peterberkenbosch wdyt?

Haven't dig it deep enough yet (tests etc). Just wanted to get some feedback confirming we could take that approach.