Add per release callbacks to Gitpan::Dist->import_releases.

Importing can take a long time, it's nice to be able to show progress.
Gitpan::Git->tag and list_tags.

Put making and listing tags into methods.

In general, encapsulate calls to Git::Repository->run as much as possible.
Protect against tag names Git does not like.

These rules are from git-check-ref-format(1).
Tag the latest from a given author and maturity level.

For #6 and #28