Add fixture overloading
Add fixture overloading

Rather than choosing one set of fixtures over another, instead load
them in ascending precedence so that all of the files are loaded when
available, but the contents of higher precedence files overlays the
data in lower precedence onces.

This mimics the strategy now employed in ActiveMerchant.

johannboutet/active_shipping,Shopify/active_shipping,Goldbely/active_shipping