bup-cron: add support for remote repository

IMPORTANT: To be able to generate the parity files, we must be able to call
bup-fsck through ssh, since it does not support remote repository by itself.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>

Conflicts:
	contrib/bup-cron
restore init functionality dropped in 74e011c
bup-cron: define a Bup class to encapsulate the calls to bup's commands

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>
reorder to follow pfrouleau
bup-cron: save some backup's stats as a git's note

The stats recorded are:
  - the size of the repository before and after the backup, and their diff
  - the version of the tools used to make the backup

nota:
  - bup-cron needs to be able to call bup, git, and python through a ssh
    session when the backup is done on a remote host, which have some
    security concerns;
  - It would be nicer if the stats functions was defined as an thirdparty
    script to be more easely customisable... but that would also be a
    security concern.

Signed-off-by: Patrick Rouleau <prouleau72@gmail.com>

Conflicts:
	contrib/bup-cron
verbose
syslog support, incomplete because we don't collect the command output
enable logfile rotation, silence stdout if logging to file

had to move the init code back before the pidfile was initialised