Simplify import ; don't inherit from Exporter

`@ISA = qw(Exporter)` is 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 just forward `->import` to `Exporter` is to use `goto`.
This avoids to pollute our `@ISA`.

Note that this patch is based on the latest code from Github, which unfortunately is older than the latest release on CPAN (see #5). I'll rebase it once #5 is resolved.