fix future exception for dropshipping

In a previous commit by yours truly, I disabled the stock-based checks
in dropshipping situations, as was suggested in #47.

The idea was correct, but the method future_orders_are_affected needs to
return False to skip the check (it means that future orders are _not_
affected, and we are cool).
bump version to 1.1
add test: for dropshipping, assume we can deliver
test dropshipping sale doesn't affect future sales
refactor common setup
add docstring to test setup, and move to the top
Merge pull request #62 from lepistone/fix-sale-exception-dropshipping

fix future exception for dropshipping