add more metadata to Makefile.PL

Add the author name, license information, and GitHub URLs to Makefile.pl. ExtUtils::MakeMaker will use CPAN::Meta (if installed) to write the data into special metadata files, and the information and links will show up on CPAN's web interface.

This is also useful for other automated tools that use this data, such as "cpanspec" (an RPM generation tool that Fedora packagers commonly use).