Add Unlicense

[gugod]

GitHub repo can be created with a "Unlicense" LICENSE file.
It would be convenient if Perl ToolChain can match that option.

[karenetheridge]

I don't think meta_name and meta2_name do not conform to the 1.4 and 2.0 META specs (https://metacpan.org/pod/distribution/CPAN-Meta/lib/CPAN/Meta/History/Meta_1_4.pod#license, https://metacpan.org/pod/distribution/CPAN-Meta/lib/CPAN/Meta/History/Meta_1_4.pod#license). (edit: specifically, meta_name should return open_source, restricted, unrestricted, or unknown, and meta2_name does not need to be defined if it would return the same as meta_name.)

We ought to have unit tests in this distribution that run through all bundled licence modules to check that they all meet the spec, so that pull requests can expose this issue through a failing test on travis.

[gugod]

@karenetheridge thanks for the review! I've correct those. In addition, I removed the "notice" section too. The usual copyright notice is not needed for Unlicense.

[Leont]

This should be updated for our new SPDX support.

[Leont]

To be merged this needs:

1. A sub spdx_expression { 'Unlicense' } method in Unlicense.pm
2. A phrase is LicenseUtils.pm to recognize such a licensed project.

[gugod]

@Leont Thanks for the review. I've added those 2 missing pieces as well as a simple t case. Hopefully this PR looks good for you.