Allow specifying search paths using environment variables.
Removed unused includes.
Merge pull request #609 from est77/master

Avoid using invalid OSL ShaderGroups.
Merge pull request #607 from est77/est_environment_searchpath

Allow specifying search paths using the APPLESEED_SEARCHPATH environment variable.
cosmetics.
minor code and formatting tweaks and cleanups.
Fixed stdosl.h OSL header.