GradeBeauty pixel engine fix for unknown channels

Patch release for GradeBeauty detailed in this PR #8

Release made with :

• a work in progress python script to help in creating release packages
• a new centos6 Linux docker image, based on devtoolset6 and cmake3

0.8 feedback

Got a few notes from the 0.8.0 release, and here are some of the changes.

• Compositors I talked to felt there was a missing plugin to complete the aovs grading stack, for a specific layer, so I added GradeBeautyLayer, and shuffled some code around so It didn't duplicate code. (could probably go further with this)
• GradeBeauty could benefit from black clamping is a user is completely removing an aov. Makes sense, since once subtracted, it is never added back.
• did some cleanup to the way documentation is displayed, went to full QT instead of relying on the os web browser if possible. This make the popup faster and more predictable.
• polished the indicator/auto label code so this will be on par with what you'd expect from a typical nuke node.
• Silenced the terminal output at Nuke startup, unless there's a problem.

0.8.0 Beta solidify pixel engines, and polish (#4)

Important milestone release

this version's plugins should be stable enough for user feedback.

compiled builds based on the work done in #4

initial release

First public version.