Fix $PATH issues with Lilypond display() hooks

By default, subprocess doesn't send it's command through a shell for processing. Among other things, this means '~' chars in the path don't get expanded. Since this is probably a common occurrence on many $PATH vars, we should use shell=True in the arguments to subprocess.

tl;dr, but if you want, see:
http://docs.python.org/2/library/subprocess.html
http://stackoverflow.com/questions/5658622/python-subprocess-popen-environment-path
http://bugs.python.org/issue8557