added all relationship methods through the sales_engine
adds all the remainins relationship methods to the associated repositories
Merge branch 'master' into invoice_relationships

Conflicts:
	lib/merchant_repository.rb
	lib/sales_engine.rb
	lib/transaction_repository.rb
	test/invoice_repository_test.rb
	test/merchant_repository_test.rb
	test/sales_engine_test.rb
	test/transaction_repository_test.rb
added relationship methods and tests for invoice: including transactions, items, invoice items, customer and merchant.
Merge branch 'master' into invoice_relationships

Conflicts:
	lib/sales_engine.rb
	test/sales_engine_test.rb
Merge branch 'invoice_relationships'