add business intelligence merchant.total_revenue and invoice.total
add business intelligence to merchant repository, most_revenue(x)
changed merchant.total_revenue to merchant.revenue
add business intelligence  merchant.total_items_sold and merchant_repository.most_sold(x)
add business intelligence to merchant, revenue by date (defaults to total)
add business intelligence to merchant repository, revenue by date
add business intelligence to merchant for favorite_customer
add business intelligence to merchant for customers with pending invoices
Merge pull request #53 from dglunz/merchant_business_logic

Merchant business logic