Add SignedJWT support and Reseller API sample to Perl Client Library.

This PR adds OAuth2 Signed JWT (Service Accounts) support to the library.

Summary:
- Add JSON::WebToken as a dependency.
- Add Reseller API sample to '/eg/service_accounts/reseller.pl'
- Add 'Google::API::OAuth2::SignedJWT' class.
- Shamelessly add myself (and email address) to Authors file.