Move "colour.RGB_Colourspace" class to "colour.models.rgb" module.
Remove "colour.plotting" from default imports.
Implement "Monte Carlo" brute force "RGB" colourspace volume computation.

Closes #158.
Implement "colour.random_triplet_generator" definition.
Refactor "colour.sample_RGB_colourspace_volume_MonteCarlo" definition using "colour.random_triplet_generator" definition.
Update default limits in "colour.volume.rgb" module.
Implement "colour.RGB_colourspace_limits" definition.
Update "colour.volume.rgb" module unit tests.
Remove intermediate variables in "colour.random_triplet_generator" definition.