Improve Zinc Axis Viewer and other tutorials

Update Axis Viewer tutorial: move application-specific code to axis_viewer.py,
add Qt Designer image and explanation, and improve flow and separate Zinc Widget explanation.
Make read_mesh tutorial use _zincwidget member for consistency.
Fix inclusion of source code in image_reader tutorial.
Explain what getting-started tutorial does.
Improve text on main tutorial index.
Merge pull request #59 from rchristie/master

Improve Zinc Axis Viewer and other tutorials