updated, refactored and cleaned up devkit packaging and releasing scripts.
added two helper hashing methods and refactored code accordingly.

one is a typed variant of foundation::siphash24(), the other one allows to combine two hashes together.
added dummy file to preserve empty directory in git repository.
fixed crash when the geometry inside an assembly that doesn't contain curves changes between two renders.
Merge branch 'master' of github.com:appleseedhq/appleseed
changed default project setup and simplified code.

frame resolution is now 1024x576 (quarter 2K, 1.78 aspect ratio) and film dimensions are 10.24mm x 5.76mm.
Merge branch 'master' of github.com:appleseedhq/appleseed
cosmetics.
fixed comments.
added a unit test.
Merge branch 'master' of github.com:appleseedhq/appleseed
minor code tweaks.
simplified code.
improved comment.
Merge pull request #522 from est77/master

Added some sample seexpr expressions.
Merge branch 'master' of github.com:appleseedhq/appleseed
expose foundation::siphash24() as a public shared library symbol.
tweaked formatting.
cleaned up Fresnel reflectance calculations.

also:
  - adjusted functions signatures to avoid unnecessary copies when dealing with voluminous spectra.
  - added a variant of foundation::fresnel_dielectric_schlick() that takes indices of refraction instead of normal reflectance.
fixed major bug in specular BTDF, and simplified code.

we were missing an eta^2 factor in the expression of refracted radiance in the non-adjoint case.