Encode: do not inherit from Exporter

Instead, import Exporter's "import" sub. This requires Exporter 5.57+
(which is in core since 5.8.3, but also available on CPAN), so
Makefile.PL is updated to add this dependency.

This removes one usage of 'use base'.
Encode::Alias: do not inherit from Exporter
"use base ..." -> "use parent"

parent.pm is a lighter subset of base.pm.
parent.pm is on CPAN, and also in perl core since 5.10.1.
Merge pull request #15 from dolmen/fix/base-to-parent

"use base ..." -> "use parent ..."