removed PDL::Pod:: classes in favor of core modules Pod::

Further changed calling implementation of overriden functions "command", "textblock" and "verbatim" and a few internal methods
Also removed StrHandle and NullHandle classes in favor of opening handles to strings
removed problematic "last" directive
removed warning about Pod::Parser incompatibility
Merge pull request #3 from PDLPorters/pod

Remove PDL::Pod:: classes in favor of Perl core modules Pod::