Added conversions for stamp types

Hi @wjwwood . How do you feel about that small change?

It allows for neater code when updating the stamp of a pcl point cloud:

compare

```c++
std_msgs::Header h = pcl_conversions::fromPCL(pcd.header);
h.stamp.fromSec( /*some double value here*/ );
pcd.header = pcl_conversions::toPCL(h);
```

or worst:

```c++
pcd.header = /*some double here*/ * 1e6;
```

with:

```c++
pcd.header = pcl_conversions::toPCL(ros::Time(/*some double here*/));
```

(I do this a lot)