Wrong namespace for `Duration` message from `std_msgs`

Hi,

The `Duration` message from [std_msgs](http://wiki.ros.org/std_msgs) is in the wrong namespace in this package.

In the file `MessageGeneration>std_msgs>Duration.cs`:
```csharp
// This should be "namespace RosMessageTypes.Std" like all the other standard messages
namespace Unity.Robotics.ROSTCPConnector.MessageGeneration
{
    public class Duration : Message
    {
```
