Small cleanup after #19, move catching of exception to process_cmd.
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.