Merge pull request #7 from infinityb/master

Fix tests
Remove deprecated syntax.

0.12.0-pre-nightly
Clean up OBJ import code.
Point-sampled image textures
Merge branch 'master' of https://github.com/gyng/rust-raytracer
Remove models submodule in preparation for move to assets repo instead.
Delete docs/models
Add assets submodule
Add skybox.
Minor CubeMap cleanup.
Avoid seams in CubeMap skyboxes.
Add more skyboxes.
Update to newest assets submodule (skyboxes).
Add bilinear filtering for ImageTexture.
Remove empty line in readme.
Add test for ImageTexture.
Super-minor comment formatting.
Early shadow exit.

~4.5% improvement on teapot scene with 32 samples. (n=1)
Fix skybox asset path in README.md