Source isnt always credit card. Fixes #26

Conflicts:
	spec/serializers/spree/wombat/customer_return_serializer_spec.rb
	spec/serializers/spree/wombat/inventory_unit_serializer_spec.rb
	spec/serializers/spree/wombat/refund_serializer_spec.rb
	spec/serializers/spree/wombat/reimbursement_serializer_spec.rb
	spec/serializers/spree/wombat/return_item_serializer_spec.rb