Fixes scattering in the ray tracing simulator

[fakufaku]

• Fixes scattering by slightly bending the rays at each reflection in addition to the diffuse rain. This follows wayverb.
• A new random sub-module that contains a Numpy random number generator to use package wide and some methods to set the seeds for this generator and that of the libroom module.
• Makes the simulator fully deterministic by relying on the package wide RNG provided by the new random sub-package.

Checklist

• Are there docstrings ? Do they follow the numpydoc style ?
• Have you run the tests by doing nosetests or py.test at the root of the repo ?
• Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
• Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".