Authen::SASL fixes

1) pass debug flag to Authen::SASL, so we can see errors
2) when auth method unavailable (for example, GSSAPI error "No credentials cache file found; output token sz:"), then fallback to another supported method
SSL and IPv6 support for Net::SMTP
added tests for Net::SMTP SSL and IPv6, save arguments in Net::SMTP to apply them on starttls
SSL and IPv6 support for POP3 incl. tests
added external test for pop3 ssl
documentation updates
Bump/set minimum Perl and EU::MM versions and overhaul Makefile.PL

Also add a .gitignore file.
Bump version for next release
Various metadata updates

- Fix missing resources from generated metadata files (the repository value
  must be a hash reference)
- Ensure minimum Perl version is stated for configure and test, not just
  for runtime
- Add previously commented-out requirements as optional features
Release 1.26
Bump version for next release
Convert tabs to spaces
Simplify Makefile.PL requirements

Requirements accumulate from one phase to the next ('configure' -> 'build'
(which also requires 'runtime' requirements) -> 'test') so there is no need
for 'test' to list requirements already listed for 'configure', 'build' or
'runtime' (or for 'build' to list requirements already listed for
'configure' or 'runtime').

(However, 'runtime' must still list everything required after installation,
but does not need to list 'perl' since 'runtime' is the phase that
'MIN_PERL_VERSION' is applied to anyway.)
Release 1.27
Bump version for next release
Merge branch 'ipv6_ssl' of https://github.com/noxxi/perl-libnet into noxxi-ipv6_ssl
Rename new test scripts from *.pl to *.t as per MANIFEST
Avoid "Name "IO::Socket::SSL::SSL_ERROR" used only once: possible typo" warnings
Avoid "Use of uninitialized value $parent in numeric eq (==)" warning when all tests skipped
Bump $VERSION in changed modules