Adding a Path getters to entity type. (Azure#60)

* Adding a Path getters to entity type.

This will reduce functionality introduced in both Azure#35 and https://github.com/marstr/azure-service-bus-go/tree/sessions

* Refactoring after feedback and investigation

After diving through the C# codebase some more, I found that the usage needed for entityPaths is actually just it's name without the AMQP connection string appended. You can see that here: https://github.com/Azure/azure-service-bus-dotnet/blob/435c25957c131c32d68b4966a1d4e83acaf01a0b/src/Microsoft.Azure.ServiceBus/QueueClient.cs#L264

I continued to keep entity.ManagementPath for convenience.

queue.go

@@ -25,6 +25,7 @@ package servicebus
 import (
 	"context"
 	"encoding/xml"
+	"fmt"
 	"sync"
 
 	"github.com/Azure/azure-amqp-common-go/log"
@@ -239,3 +240,7 @@ func (q *Queue) ensureSender(ctx context.Context) error {
 	}
 	return nil
 }
+
+func (e *entity) ManagementPath() string {
+	return fmt.Sprintf("%s/$management", e.Name)
+}