Ensure consistency of backup in case of failure

 - Things have changed a little. cur is now only a symlink to the last
   timestamped subvolume.
 - A temp subvolume is created while the rsync is running.
 - Only in case of a successful backup do we create a timestamped
   subvolume and update cur symlink. Fixes #5.
 - Remounting of containing parition is not a responsibility of this
   backup script and so it was removed. This means first argument of
   init and backup commands are no longer needed.

Signed-off-by: Mansour Behabadi <mansour@oxplot.com>