Define ProductPrioritySet interface
Remove TestUnit
Create ProductProritySet module with its ProductPriorities table and ProductPriority model to control next batches of product to be queried on Amazon
Define ProductPrioritySet send_next_batch method to be called by a Worker
Add model tests for ProductPriority class
* Add factory for Product Priority
* add 'add_to_set' test for ProductPrioritySet module
* Add delete_persisted_product_priorities test for ProductPrioritySet
* Add factory-girl product hash
Add ProductPrioritySet next_batch test. NOTE: gives deprecation warning for #count method
Merge master branch with current branch
* Updated labeling of sim_products to correct asins_of_sim_prods to match model
* Add persist_products test for ProductPrioritySet module
Add handle_new_products tests for ProductPrioritySet module
* Correct association assignment for product if product already exists
* Add two more tests to evaluate relationship of parent product and similar product
Merge with mastser branch