added transaction relationship methods and tests
modified the relationship methods to call find_merchant_invoices from sales_engine so that the invoice repository looks up invoices by merchant id not customer id
merging to incorporate latest updates

Merge branch 'add_merchant_relationship_methods' into add_relationship_methods_to_transaction_and_repo
modified relationship methds to call find_transaction_invoice from sales engine so that invoice_repository returns an invoice by id
Merge pull request #60 from oorion/add_relationship_methods_to_transaction_and_repo

added transaction relationship methods and tests