Remove boost::make_shared invocations.
Add depreciation macros.

Old pcl::io::savePNGFile() functions are obsolete now, as well as the
organized_pcd_to_png tool.
Use new pcl::io::savePNGFile() function.

This gets rid of depreciation warning in example_supervoxels.cpp and
organized_pcd_to_png.cpp.
Make "rgb" field the default option in pcd2png.

This way the default behavior is the same as of the organized_pcd_to_png
tool.
Add random color mode for "label" field.

This allows to get a colorful image where different labels are painted
with random colors.

Unit tests and pcd2png tool are updated accordingly.
Add more documentation for PointCloudImageExtractor.