added invoice method that takes id_from_invoice to
get the invoice item’s invoice
refactored invoice to make it more clear what’s calling where
add new functionality for lower level classes
added tests for new methods
refactored invoice for clarity
add methods to support new functionality
Refactor customer invoices relationship #61
Refactor transaction invoice relationship #61
Merge branch 'resolve_merge'

Conflicts:
	lib/sales_engine.rb