Specify the default Perl::Critic configuration

Without this, Perl::Critic will search for a .perlcriticrc file in the
user's home directory or via the PERLCRITIC environment variable (in the
absence of our own one in the current directory), which may cause test
failures (as seen on CPAN Testers) if a non-default policy configuration
is set.
Skip Perl Critic, Pod and Pod Coverage tests unless AUTHOR_TESTING

Even after telling Perl::Critic to use its default policy configuration
we may still see unexpected failures from CPAN Testers resulting from
different versions of Perl::Critic having different policies.

Pod tests may likewise have unexpected failures caused by users having
different versions of the testing modules to the author.

None of these tests are relevant to users anyway, so disable them by
checking for $ENV{AUTHOR_TESTING}, as per the Lancaster Consensus.
Reformat Changes file as per CPAN::Changes::Spec.
Add optional Changes testing (skipped unless AUTHOR_TESTING).
Minor edits to Changes file format