autodie::exception::system: "use base" -> "use parent"

Because parent.pm is lighter than base.pm, is also in core since perl
5.10.1, and is available on CPAN for older perls.
autodie::hints: drop Exporter inheritance

... because we don't export anything!
t/lib/*.pm: "use base qw(Exporter)" -> "use Exporter 5.57 'import'"

We don't need to inherit from Exporter. We just need its 'import' method
to be able to export. This is possible since Exporter 5.57 bundled with
perl 5.8.3. This distribution depends on perl 5.8.4 so this doesn't add
a dependency.
t/*.pm: s!use base!use parent!
Merge pull request #59 from dolmen/base-to-parent

Replace "use base" with "use parent"