Added support for EOF and ^D and environment variables to headless_piano...

This change makes scripting easier, because you can do something like "headless_pianobar <<< p" to pause, without having the program hanging out there. I hope to add support for command-line arguments in order to do something like "headless_pianobar p".

I also thought some might appreciate the environment variables, although I don't actually have much of a use for them. I am sure there is a way to do that more succinctly, but I don't know what it is.