Clarify licensing, ensure footers are consistent throughout the project

Note - this *DOES NOT CHANGE* the licensing of the project in any way, it
is still dual-licensed like perl5, just as it was before this commit. All
additions are for clarifying purposes only.

The way licensing is presented is an amalgamation of information and
prior art found at the following sources:
 - http://www.perlfoundation.org/cpan_licensing_guidelines
 - https://metacpan.org/source/ETHER/Moose-2.1212/LICENSE
 - `git log --color -w -p v5.20.0 ^perl-5.000^ -- Artistic`
New Pod::Inherit does not need Pod::Tree
(travis) Attempt to grab more info on parallel_installdeps_notest failures
After 5268b1da populate remained the sole user of Data::Compare

Might as well throw it out - the check that takes place is rather rare and
a freeze-and-compare approach is likely more efficient anyway