Add bass and treble

Adds functionality like we already have for volume to bass and treble.

Requires merge of #14 for the moving of socos.py into socos/core.py, it is built on top of that branch, so look at the latest commits for differences between the two, or just download both and do
```git log -p --pretty=oneline --abbrev-commit fix_illegal_seek_target..add_bass_and_treble```

I haven't looked at balance yet, but I assume it can be done in the same way.

I'm sure there's more ways to clean up and optimize the new MusicAdjustments class, and I'm highly interested in feedback on this.