Don't inherit from Exporter

« @ISA = qw(Exporter) » was only used to allow to call
$class->export_to_level(1) in ->import. This is a way to tell Exporter
to do its usual job.
But a simpler way to to just forward ->import to Exporter with "goto".
This avoids to pollute our @ISA.
Merge pull request #7 from dolmen/fix/dont-inherit-from-Exporter

Simplify import ; don't inherit from Exporter