Add support for the RunScript directive in Job definitions

For example:

bacula::job { 'foo':
  files => ['/bar'],
  runscript => [
    { runs_when      => 'Before',
      runs_on_client => true,
      command        => 'sh -c "/bin/date >> /tmp/date.log"' },
  ]
}

This will also allow mere mortals without the enterprise mysql/postgresql plugins for Bacula to dump a database before running the Job that backs up the dump.