Fixed leftover cosine term, reference images.

        - Fixed left-over cosine term in final accumulator expression.
        - Re-rendered reference images, updated test scenes.
Fixed potential negative pdf values.

        - Fixed potential negative pdf values when evaluating the BRDF
          response, one of the cos(phi_diff) factors was not absolute.
Updated scenes, added reference images.

        - Removed old reference images.
        - Updated ONW scenes and reference images.
Remove asserts, cleanups.

        - Remove asserts on C1,2,3 coefficients, remove minimum of
          outgoing angle, fixed absolute value on cos_phi_diff.
Added assertions to the 2nd and 3rd coefficients.
Merge branch 'master' of https://github.com/appleseedhq/appleseed into oren_nayar

Conflicts:
	src/appleseed/renderer/modeling/bsdf/orennayarbrdf.cpp
Merge pull request #421 from luisbarrancos/oren_nayar

Oren-Nayar fixes and cleanups