Added merchant.revenue method
Added Merchant.most_revenue(X) method and test
refactored merchant.revenue method
Added method: merchant.customers_with_pending_invoices
Added method for merchant.revenue(date)
working on daily revenue merchant feature
merging updated master to branch
Added method for find total revenue of all merchants by specific date
Added method for finding a merchant's favorite customer.
Added Merchant.most_items(x) method
Implemented BigDecimal for revenue outputs