In your system preferences, select the application to start up on login in the "Users" preference pane. Now everytime you log in, the app will open.

When the app starts up everytime, a status window will appear to tell you the app successfully launched.

Because of the way I wrote the app, there will be no dock icon and it will not show up in your "Force Quit" items. THe only way to quit the app is to logout or to Kill Process using Activity Viewer.

Once the status window comes up, you can click on it and press CMD+H to hide the window. It is now effectively "running in the background". I don't know how much more "in the background" I can make it. I like having the status window initially come up because if the script isn't working you wouldn't be able to troubleshoot it.