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Bug fixes and refactoring away from dependencies

Since we've moved a long way from datashift-spree's approach, I feel we should be treating our version as a distinct product. To this end, I've started removing dependencies on datashift. I see the eventual outcome as a simpler gem designed specifically for importing products into Spree, rather than a complex one built on a general-purpose framework.
- moved some methods from datashift
- renamed load_object to product
- separated code in process for handling special cases into own methods
- fixed issue with dimensions missing from some variants