Use fancycompleter for completions

This allows using [fancycompleter](https://pypi.python.org/pypi/fancycompleter) as the completion mechanism, which offers a few nice features, the most noticeable being that it keeps the completions on the screen and filters them down as you type more characters. The main selling point of fancycompleter is color, but I haven't made any use of that yet. I played around a bit with trying to color the group namess green and the individual services purple, but it was buggy so I took it out. Perhaps more useful coloring for supervisorctl is to show stopped services in red and running services as green.

It's possible to turn off fancycompleter by setting an environment variable `SUPERVISORCTL_NO_FANCYCOMPLETER ` -- this will make it use the regular readline completer.