Add OrdersController spec to deal with payments redirection
Method for total cost of Order calculation
Bulk of factories for models objects creation
Change with approach to payment response validations

Thanks to integration tests found mistake with Order states. Previous assumption that Order with status 'open' is processable was wrong. Changed to valid status: 'sent' which means that payment process has been initialized.
Moreover changes with data conversion introduced.
Minor tests improvement
Integrate payments response processing actions.
Introduce PaymentLogger responsible for storing payment callbacks data
Refactor Payments#create as service object
Improve comments and test description
Change approach to few associations dependencies
Improve item prices calculation
Replace old Hash notation with new one
Refactorization of Orders#create action with resp. delegation
Add foreword with solution description
Remove RSpec warnings from tests output
Remove .keep files
Bulk of comment improvements
Change action responsible for payment request processing

Previous version assumed that payments service sends POST with appropriate params as payment process confirmation. It appeared hard to maintain and test. Morever previous solution needed more tricky code (e.g. disabling CSRF protection). Changed routing from POST /payments to GET /payments/finish. Removed requests spec as redundant relative to feature specs.
Update foreword about changes in design