Add erase parameter to Block.Format() to zero out device before formatting it

Add erase parameter to Block.Format() to zero out device before formatting it

... and also provide new job properties so it's possible to build a
meaningful UI around it. This makes things like this possible

 http://people.freedesktop.org/~david/gnome-disks-erase-overwrite-option.png
 http://people.freedesktop.org/~david/gnome-disks-long-running-operations.png

Signed-off-by: David Zeuthen <zeuthen@gmail.com>