Market

- refactoring: `Cash() (float64, error)`
- add: Equity() (float64, error)

market.go

@@ -28,6 +28,7 @@ type Market interface {
 	Orders() ([]Order, error)
 	Portfolio() (Portfolio, error)
 
-	AvailableCash() (float64, error)
+	Cash() (float64, error)
+	Equity() (float64, error)
 	MaxBudget() float64
 }