Add functionality for showing and adjusting bass, treble.

Move music adjustment related functionality to separate class.

Builds on and requires https://github.com/SoCo/socos/pull/14 for moving
socos.py into socos/core.py
Import print_function for backwards compatibility.
Renamed class MusicAdjustments to RenderingAdjustments.
refactor volume/bass/treble adjustment
run doctests from makefile
Merge pull request #34 from SoCo/vol-bass-treble

Add bass and treble (refactored)