Add auto-start option to prefs dialog

In order to make enabling AWN to automatically begin with each session. An auto-start option could be placed in awn-manager. This would make it so the user would not have to manually configure their session to start awn. Other programs, like screenlets-manager, have implemented this and could be looked at for examples. Both screenlets-manager and awn-manager are written in python.

In Ubuntu Gnome at least, adding a command to the start up programs list in the session manager seems to simply add a desktop file to ~/.config/autostart

It shouldn't be to hard to implement a check box which creates an awn desktop file in that location when checked and deletes it when unchecked.