Adding Octree code from jnphilipp

Repo with original code: https://github.com/jnphilipp/Raytracer
                         git@github.com:jnphilipp/Raytracer.git
Modify foreign code. Texture mapping. Basic raytracing.
Update README.md

Add note for foreign code.
Compilation optimization flags

Big performance boost with 'release' version and -march=native.
Slightly better with -03.
Since I'm not debugging ( no debugger :( ) it is better for fast testing.
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