Add an overload of pcl::io::savePNGFile().

The new function takes sensor_msgs::Image and saves it as a PNG file.
Currently supported are "rgb8", "mono8", and "mono16" image encodings.
Add PointCloudImageExtractor base class and several implementations.

* PointCloudImageExtractorFromRGBField produced a color image based on
  the data in "rgb" field of a point cloud.
* PointCloudImageExtractorFromNormalField produced a color image based on
  the data in "normal" field of a point cloud.
* PointCloudImageExtractorFromLabelField produced a mono image based on
  the data in "label" field of a point cloud.
* PointCloudImageExtractorFromZField produces a mono image based on the
  "z" field of a point cloud.
* PointCloudImageExtractorFromCurvatureField produces a mono image based on the
  "curvature" field of a point cloud.
* PointCloudImageExtractorFromIntensityField produces a mono image based on the
  "intensity" field of a point cloud.
Add an overload of pcl::io::savePNGFile().

The new function takes a field name and uses appropriate
PointCloudImageExtractor subclass to extract an image from the given
field. Currently supported are "normal", rgb", "label", "z",
"curvature", and "intensity" fields.
Add pcd2png application to the "tools" module.
Add tests for Point Cloud Image Extractors.
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.
Merge pull request #204 from taketwo/png-io

Add a generic `savePNGFile()` function