Refactor Checkout

* Remove the inverse dependence on nereid_payment
* Use the payment_gateway modules
* Allow using payment profiles and storing credit card info
Merge pull request #18 from sharoonthomas/develop

Refactor Checkout
Bump version to 3.0.2.0dev1
Fixed flake8 issues
Merge pull request #20 from GauravButola/feature/task-4203

Fixed flake8 issues #4203