png2pcd: convert color and depth image to a pcd file

I use rgbd dataset from http://rgbd-dataset.cs.washington.edu/dataset/rgbd-scenes/ to test this.
Color image:
![desk_1_10](https://cloud.githubusercontent.com/assets/1129415/3013762/e9409f2e-df4d-11e3-8105-704f345c0930.png)

Depth image:
![desk_1_10_depth](https://cloud.githubusercontent.com/assets/1129415/3013786/66effa50-df4e-11e3-9a7d-608210cd04dc.png)


Gray image:
![desk_1_10_gray](https://cloud.githubusercontent.com/assets/1129415/3013770/fe890fb0-df4d-11e3-93e6-2a60eb03519b.png)

Test case 1: color + depth ->  pcd(color)

    pcl_png2pcd desk_1_10.png desk_1_10_depth.png out.pcd --intensity_type FLOAT

![pcd viewer_003](https://cloud.githubusercontent.com/assets/1129415/3013805/adf44000-df4e-11e3-8d7e-6e36516c6dab.png)


Test case 2: color + depth -> pcd(force gray)

    pcl_png2pcd desk_1_10.png desk_1_10_depth.png out.pcd --intensity_type FLOAT -mode FORCE_GRAYSCALE

![pcd viewer_004](https://cloud.githubusercontent.com/assets/1129415/3013809/bd6dc7c2-df4e-11e3-9064-5b5e1f4e7265.png)


Test case 3: gray + depth -> pcd(gray)
    pcl_png2pcd desk_1_10_gray.png desk_1_10_depth.png out.pcd --intensity_type FLOAT

![pcd viewer_006](https://cloud.githubusercontent.com/assets/1129415/3013815/d7162a98-df4e-11e3-955f-bbf5476ac801.png)

Test case 4: gray + depth -> pcd(force color)

    pcl_png2pcd desk_1_10_gray.png desk_1_10_depth.png out.pcd --intensity_type FLOAT -mode FORCE_COLOR

![pcd viewer_005](https://cloud.githubusercontent.com/assets/1129415/3013813/c636bb70-df4e-11e3-95ff-1a52e9a9aa3d.png)