Set RGB color handler as default in pcl_viewer

pcl_viewer will use random color handler by default. It looks like this:
![pcd viewer_002](https://f.cloud.github.com/assets/1129415/2332084/648b59a8-a452-11e3-8ba5-58e0b0a121ec.png)
However, RGB color handler is more frequently used and we need to press keys to switch to the right color handle. So set it as default:
![pcd viewer_001](https://f.cloud.github.com/assets/1129415/2332172/7e13d566-a453-11e3-9c25-6a64b0fc13bb.png)