Fixing tagmanigest-md5.txt generation

In 0.046 I noticed the is an issue with the generation of tagmanifest-md5.txt files using:

      use Archive::BagIt;
      my $bag = Archive::BagIt->make_bag('directory/bla');

This will create a tagmanifest-md5.txt with content:

      d41d8cd98f00b204e9800998ecf8427e  tagmanifest-md5.txt

Manifest files can't contain an entry about themselves, plus the bagit.txt, bag-info.txt and manifest-md5.txt are skipped.