Added Oren-Nayar BxDF, closure.

        - Added Oren-Nayar BRDF, OSL closure.
Updated cmakefiles, OrenNayar.
Added OrenNayar test scenes.
Added OSL OrenNayar shaders.
Added Oren-Nayar OSL shaders.
Added Oren-Nayar OSL test case.
Registering Oren-Nayar to OSL.
Registering Oren-Nayar to OSL, fix typo.
Updated OSL test scene, cleanups.

        - Updated OSL test scenes for Oren-Nayar BxDF.
        - Removed unused variables, minor fixes and cleanups.
Merge branch 'master' of https://github.com/appleseedhq/appleseed into oren_nayar

Conflicts:
	sandbox/tests/test scenes/osl closures/01 - area light - ptne.appleseed
Added Oren-Nayar to BxDF test scene.
Formatting fixes.
Fixed formatting, added test scene, references.

        - Skip std::pow, store C2 coefficient in a temporary variable
          and use multiplications instead.
        - Fixed formatting issues.
        - Added second test scene for the BSDF, showing response at
          grazing incidence. Added respective reference images.
        - Minor cleanups and fixes.
More OSL transparency work.
Execute OSL shaders when computing back lighting.
Fix MinimizeButton minor issues

- sign sources
- keep sources alphabetically in CMakeLists.txt
Added transparency scene.
OSL transparency in shading engine and path tracer.
Merge pull request #401 from marius-avram/B331-sign

Fix MinimizeButton minor issues
Exec OSL shaders in tracer, plus some refactors.