v0.6.0 - Maintenance release

This is just a maintenance release, almost four years after v0.5.0.

PythonQwt is still alive... and it's still running!

After being widely used through guiqwt which extends PythonQwt features (adding higher-level features like efficient image visualization, advanced interaction with graphic objects, ready-to-use GUI components to build your scientific application with a few lines of code, etc.), PythonQwt may also be used through plotpy.