Added headless mode and check if PMS is running

I'm sorry, I am not very familiar with Git yet and I tried to create two different pull request, but it will only let me do both changes at once.

First one allows us to run PlexConnect via cron or launchd without throwing an error, but still allows for shutting it down while running in Terminal.

The second change is a check if Plex Media Server is running. If it is not, wait for it to start before doing any DNS stuff.