Moved modifiers to Box'd types. Closes #8.

As a result of this change, NoiseModules now implement the Clone
trait (except for FunctionNoise, which throws an error).

This commit also contains some minor documentation fixes, marks for
stability, and scaling for PinkNoise to bring its output into the
[-1, 1] range.

Signed-off-by: Atheriel <atheriel@gmail.com>