Updated the "Surface Classes" section.

I updated both the text and images for the "Surface Classes" section to
be compatible with the the newest version of CellBlender (0.1.57).
Links to context button images for Blender 2.67.

The context buttons changed slightly in Blender 2.67. The images for the
Scene and Object context buttons were updated in the previous commit,
but the references to them in "Getting Started" needed updated.

A section about saving the project was added to the end.