Changed data structure of purchase_request.rb

@@list and @@completed_list, which stores the purchases_request is no longer a hash and instead is now instances of the PurchaseRequestRepo.
Added property 'status' to each  purchase requests.
self.complete method now changes the status from 'pending' to 'completed'
Added method self.get_completed_forms which retrieves all completed purchase requests.
Added tests retrieving completed purchase request.
Passed on tests on purchase_request_spec.rb