fixed an import error in order and added tests
Merge pull request #40 from Shopify/fix_order_import

fixed an import error in order and added tests
fix for issue 38

* I used from .model import Model, using .
* solved some circular import with product
* added extra Pep 8 formatting that my editor warned for
#38 added imports according to the rest of the project.
#38 missed on file also remove . for import
Merge branch 'branch_for_issue_38' of github.com:acclivity/shopify_python_api into branch_for_issue_38