Switch from File::Spec to IO::Spec

Stop depending on external File::Spec, since IO::Spec is in the setting.
Fix octal 0 bug in PB::Actions
Add a stub PB::Message to prepare for generation

Generated message classes will need a common superclass, so stub out a
PB::Message for this purpose.
WIP: Message class generation using Perlable*

Start generating message classes in PB::Model::Generator using the new
Metamodel::Perlable* framework.  Incomplete and will need some refactoring,
but should be enough to start work on writing complete messages in
PB::Binary::Writer.  Some debug code still included for now.