add GitHub repository to MakeFile

the link will appear in metacpan side bar if present in MakeFile.
Condition to check for required ExitUtils::MakeMaker version used the
same way you did for the other stuff
add license information

Hi! Having to look at the README file for license is tedious. What about
adding it to the POD? Many other modules do it, too.
Also, the kind of license can be added to the MakeFile. This way it
shows up correctly in the metacpan side bar.