Ensure all subprocess.Popen args are strings and fix small typo

When running `Notifier.list(os.getpid())` (as per the documentation example), I get the following error:

    File "/System/Library/Frameworks/Python.framework/Version/2.7/lib/python2.7/subprocess.py", line 1228, in _execute_child
    raise child_exception
    TypeError: execv() arg 2 must contain only strings

Apparently `subprocess.Popen()` takes only string arguments. Line 67 ensures this. Also fixed a small typo "sublitle" -> "subtitle".