Set RGB color handler as default in pcl_viewer
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