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.
Merge pull request #1 from ktdreyer/metadata

add more metadata to Makefile.PL