updated to accept a repository
add attr_reader for repository in invoice

added test to assert invoice has a repository
Removed unnecessary @ signs because it has an attr_reader for item
removed unnecessary @ signs because we have an attr_reader
Changed merchant to accept a repository

Changed merchant_test to test that merchant accepts a repository

Changed merchant_repository to load a file and accept a sales_engine

Changed merchant_repository_test to test that merchant_repository can load a file.