Add shell version of powerline-client

Note: this shell script is still faster then client/powerline.py. `time` reports 
      0.00, 0.01, 0.02÷0.03 for powerline, powerline.sh and powerline.py 
      respectively (run with `./client/powerline* tmux right`).

Note: does not work in busybox as env there does not support -0 argument. 
      Requires sed, id (for dash as $UID is not available there), env, printf 
      and socat. Is not currently handled by setup.py.