Allow resuming from an existing repo.

The tests always deleted the gitpan directory and never had to deal with this.
Allow a Gitpan::Dist to continue working with an existing repository.

The tests always blew away the repository directory between tests of the
same distribution.

Don't trust that just because there's a repository directory it's a valid
repository or in a good state.
Finally, a class to put it all together and import sets of dists from BackPAN.
Fix gitpan-import to work with Gitpan.pm.

Removed a lot of the options.  Many have moved to the config file.
It works, ran it against all the Acme distributions.
Add the dependency on Parallel::ForkManager needed by Gitpan.pm.
Make it official, this is (will be) 2.0.0.