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)
Merge remote-tracking branch 'author/feature/daemon'