Add 'dry_run' option (-d) and 'ignore __buildout_signature__ difference' option (-S).

This commit adds two following options ::

  -d

    Dry-run mode.  With this setting, buildout will display what will
    be uninstalled and what will be installed without doing anything
    in reality.

  -S

    Ignore __buildout_signature__ difference in comparing with the
    previously installed state.