add SimpeCircleFacingVisualizer class
Merge remote-tracking branch 'refs/remotes/origin/master' into garaemon-add-simple-circle-facing-visualizer
Merge remote-tracking branch 'refs/remotes/origin/master' into garaemon-add-simple-circle-facing-visualizer
Merge pull request #136 from garaemon/add-simple-circle-facing-visualizer

refactor code of TargetVisualizer