[#4183] Implemented HourlyDeliverable#profit_left.

app/models/hourly_deliverable.rb

@@ -53,4 +53,10 @@ class HourlyDeliverable < Deliverable
     budgets = labor_budget_total + overhead_budget_total
     (total || 0.0) - budgets
   end
+
+  # The amount of money remaining after expenses have been taken out
+  # Profit left = Total - Labor spent - Overhead spent
+  def profit_left
+    total_spent - labor_budget_spent - overhead_spent
+  end
 end