Merge pull request #13 from MountainRoseHerbs/2-3-stable-mrh-fix-sku-generator

Fix factories to generate unique SKUs
Refactor object graph access to deduplicate loads

Reduces the available object graph walks to access Spree::Adjustment
instances to one.

Commit will still be amended with more details. Still WIP. Just backing
it up on GH this way.