Add autostart_usb.py to force a usb-drive to be present

[andyboeh]

This adds a supplemental script to use instead of autostart.sh that waits for a USB drive to be plugged in. The configuration is adapted so that all pictures are stored on the thumb drive. If a file called "background.jpg" is present, the config is changed to use it as background image. If a photobooth.cfg file is present, all keys are merged with the photobooth.cfg file present on the system. Attention: The files are saved, I use it with a read-only root filesystem that uses a temporary unionfs for storing files. It also adds a simple splash screen while loading.