Added --help and --unlogged options

- Added --help / --h / --usage / --? options to output the "help" using pod2usage.
- Added "UNLOGGED" to the "_pg_sample" table creation commands to reduce the overhead of creating the tables. Could also add in a "TEMPORARY" option to further reduce the impact on the server.
- Added a check for the postgres version (from "pg_dump --version"). It's a bit hackish, but I don't know how else to do it.

Note: I am not a Perl coder, I pretty much just followed what you already had written and tried to be consistent.