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
Merge branch 'master' of https://github.com/PointCloudLibrary/pcl