convert color and depth image to a pcd file

Use pcl_png2pcd to convert color and depth image to a pcd file.
Example:
  pcl_png2pcd color.png depth.png out.pcd --intensity_type FLOAT
Tested with RGB-D dataset from
http://rgbd-dataset.cs.washington.edu/dataset/rgbd-scenes
Merge pull request #680 from xiangxw/powerful_png2pcd

convert color and depth image to a pcd file