Merge pull request #630 from est77/master

Fixed bug in ProjectFileWriter python binding
added comments.
major overhaul of the non-physical lights subsystem.

- the physical Sun's is (finally) fully functional.
- fixed directional light when instantiated in scaled assemblies.
- fixed multiple instancing of point, spot, directional and sun lights.
- renamed inputs of point, spot, directional and sun lights according to the actual physical quantities they represent.
added test scenes and updated test suite reference images.
Merge branch 'master' of github.com:appleseedhq/appleseed
fixed a small bug in directional and sun lights, plus many code optimizations.

we were using m_scene_radius instead of m_safe_scene_diameter when computing ray origins in the sample() methods.
renamed a test suite.
updated .gitignore.
updated unit test data.
fixed a bug in updateprojectfile.

the project file format migration from revision 7 to revision 8 would fail.
improved runtestsuite.py tool.

it now prints the shell commands to update reference images.
updated all test scenes to project file format revision 9.
code simplifications and cleanups.