Delete import products

[sbounmy]

Hi Joshua,

Along with https://github.com/joshmcarthur/spree-import-products/pull/16

I need admin to be able to 'rollback' imports if anything goes wrong so I basically implemented it for my own usage, if you don't feel the usage of this feature, I totally understand :).

This pull request :

• set the import process in a SQL transaction so we can rollback when an exception occurs : prevent half created importation.
• allows admin to track what product an import have been created
• admin can delete import which destroy associated products

This PR is quite long but has model and integration tests aswell, let me know if you have any questions.

[joshmcarthur]

I'll have to think about this one.

Things I like:

• Tests
• Wrapping the import in a transaction

Things I'm not sure about:

• Adding a whole new way of managing this import - I think it's cool, and it's something I've considered as well, but it's still a pretty big change. I'll have to have a think on this point.

[sbounmy]

Thanks for your feedback @joshmcarthur !

Sure, take your time and let me know if you have any questions !