Add support for listing services, very rough

registry/registry.go

@@ -4,6 +4,7 @@ type Registry interface {
 	Register(Service) error
 	Deregister(Service) error
 	GetService(string) (Service, error)
+	ListServices() ([]Service, error)
 	NewService(string, ...Node) Service
 	NewNode(string, string, int) Node
 }
@@ -23,3 +24,7 @@ func Deregister(s Service) error {
 func GetService(name string) (Service, error) {
 	return DefaultRegistry.GetService(name)
 }
+
+func ListServices() ([]Service, error) {
+	return DefaultRegistry.ListServices()
+}