New: trajkovic and hedley detector 2D
New: port of Trajkovic and Hedley detector to pure geometry
Fix: a silly bug in normals diff computation
Add: pcl_ni_trajkovic application to detect Trajkovic 2D/3D points from OpenNi device
Fix: removal of unused argument and normals_->resize statement
Add savePNGFile function for histograms.

Render the histogram using vtk offline and then save the image to png
file.
Add: pcl::BadArgumentsException class to be thrown from namespace

For void free functions when arguments are wrong throw this exception
Add function to compute interpolated index from current index, length and border type
Update ply_io.cpp

Reading a binary PLY file with faces and "vertex_indices" property crashes, if only a PointCloud is provided. Since the polygons_ member is set to 0 and untouched otherwise.

Worked in PCL 1.6, doesn't work right now with 1.7.1.

The ply2pcd sample program provided with PCL crashes as well.
Set RGB color handler as default in pcl_viewer
Fix warnings in filters/grid_minimum.h

> grid_minimum.h:83:9: warning: field 'divb_mul_' will be initialized after field 'min_points_per_grid_' [-Wreorder]
Register PointXYZLAB point type properly

This commit adds an auxiliary point type `_PointXYZLAB` (similarly to
how it is done for native PCL point types) in order to fix the invalid
offset warning:

> gicp6d.h:78:1: warning: offset of on non-POD type 'pcl::PointXYZLAB' [-Winvalid-offsetof]
Add a draft of contributing draft
Merge pull request #561 from taketwo/fix-warnings

Fix compiler warnings
Merge pull request #562 from taketwo/add-contributing-guide

Add a contributing guide
Rearrange sections in CONTRIBUTING.md

Swapped "Pull requests" and "Bug reports" sections to give the former
more priority, deleted "Suggested process" section (moved it to the
project Wiki).
Add "License" section to CONTRIBUTING.md
Merge pull request #566 from taketwo/update-contributing-guide

Update contributing guide
Merge pull request #556 from xiangxw/master

Set RGB color handler as default in pcl_viewer
Add: morphological filters operate on the z dimension (replaces #521)