[setup.py] Update optional plugin dependencies

echonest and mpdstats plugins need pyechonest and python-mpd respectively so
let's list them properly
Back out a changelog entry

I usually don't add changelog bullets for project metadata---maybe this is
misguided, but it's nice to have everything be relevant to how beets itself
works so the changelog doesn't get too boring. I also don't list documentation
improvements, for example.