Complete rewrite of DD.pm.

Simplify. Code is much shorter.
DD now only inherits from Data::Dumper. It still gets Exporter features
as Data::Dumper inherits from Exporter.
Fixes issues such as DumperX (from EXPORT_OK) not being imported by
importing @Data::Dumper::EXPORT_OK.
'D' becomes just an alias of 'Dumper' instead of being a dummy wrapper.
Makefile.PL: Fix dependencies

TODO: add more metadata (resources...)