Set matplotlib backend globally
Create matplotlib config directory
Set matplotlib qt backend
Avoid import of matplotlib QT lib if QT is not installed
Use None instead of 'none' for qt backend
Fix setting of QT_API environment variable
Fix python 3 print statement in example script
Fix window size of sub-pixel corner detection
Fix matching example script
Merge pull request #571 from ahojnnes/travis-fix

Fix: Use None instead of 'none' for qt_api (fix Travis error), set matplotlib backend
  also use python 3 print() in example.