add ContributorsFromGit dzil plugin

This will add the list of contributors to the project -- as divined from the
authors of the reachable commits -- to the x_contributors metadata tag.  Other
tools and sites, like http://metacpan.org use this information to list
contributors.
Merge pull request #36 from RsrchBoy/topic/contributors-metadata

add ContributorsFromGit dzil plugin