Support for backup directory parameter -Fd in order to enable multiprocess pg_dump

[DanScharon]

This is a followup issue regarding #12

Currently the parameter -Fd for pg_dump is not supported by autopostgresqlbackup. But it is a prerequisite for the parameter -j which enables the use of multiple processes in order to speed up the dumping. This is only possible if the dump target is not a file but a directory.