Add files from @kovidgoyal's powerline-daemon repo

Minor changes have been applied:

- Removed copyright info and GPL 3 license since Powerline is MIT (needs
  confirmation from kovidgoyal before merge)
- The `powerline-client` script is renamed to `powerline`, and calls the
  daemon or `powerline-render` (the previous `powerline`) to render
  a statusline
- Minor coding style corrections to match the rest of the project
- Python 3 support is removed for now due to setuptools failing with
  binary scripts

Todo:

- Automatically attempt to launch powerline-daemon the first time
  powerline is run if the daemon isn't already running?
- pip install -e fails with binary files (it appears that pip recodes
  the powerline binary to ASCII, the compiled powerline script must be
  copied manually to ~/.local/bin after pip install -e has been run)
Use data_files on python-3
Use powerline/powerline-render pair in place of powerline-client/powerline
Do not use scripts=[] to install powerline at all.
Add a note about not fully working --editable to linux.rst
Get rid of modeline E518 error
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.
Remove compiled binary from repository
Fix UnicodeDecodeError in Python version of powerline client
Some style fixes
Show exception in setup.py and try to fall back to powerline.sh
Add `powerline-config shell command` and use it in all shell bindings
Also use the same code for tmux
Make daemon work with aboveleft shell key
Run shell tests with and without daemon
Fall back to powerline-render when using powerline.sh
Do not hardcode scripts/powerline in input.*
Fix zsh daemon tests: disable hostname and user before testing select

Reason: hostname and user do not have stable width and select prompt width with 
daemon depends on previous prompt width.
Check whether daemon log is as empty as it should
Add powerline-daemon --quiet