Make guess_license_from_meta support CPAN::Meta::Spec 2.0

This takes into account the fact that the license field in 2.0 is an array, by simply allowing a "[" character before the license. Sloppy, but that seems the best way short of adding full YAML/JSON parsing to the module. (A test case is provided for this.)

It adds support for the new license strings defined in 2.0, such as "perl_5".

And it adds a new method `guess_license_from_meta_code` (you might want to change the name?) which bypasses the YAML/JSON parsing stuff.