updated sales_engine to load test data

moved test data to new folder inside data folder
removed spaces on engine

added test for invoice
fixed trailing whitespace that magically appeared
removed @ signs that weren’t needed because of attr_reader
refactored to decrease line length
Merge branch 'master' of https://github.com/trayo/sales_engine into add_better_tests_for_sales_engine
shortened file_loader cause it didn't need a returning variable
Merge pull request #78 from trayo/add_better_tests_for_sales_engine

Add better tests for sales engine