Added usb storage

Add the ability to copy your backup to a USB drive for taking it offsite. I've included some tests.

You do have to put the following in /etc/fstab for this to work and create the mount directory under your user.

    /dev/disk/by-label/BACKUP	/home/youruser/usb	  auto	rw,noauto,user,exec	0	0 


An example of how I use it in my backup scripts

    store_with Usb do |usb|
       usb.path = "/home/map7/usb/test"
       usb.usb_mount = "/home/map7/usb"
    end