after some thought, realized the 'nthash' name should implement
the raw hash. since bsd_nthash was very rarely used, it shouldn't
present backwards incompatibility issues at this point to go
ahead and rename it.

* new utils.Base64Engine() provides flexible class for encoding arbitrary base64 charmaps.
  it should also be a bit faster than the old h64 module.
* predefined 'h64' instance has mostly the same methods as the old h64 module
  which it takes the place off - so imports should be unaffected.
  (the only the exception of the xxx_dc_xxx methods, which now use the 'h64big' instance)
* replaced utils._blowfish base64 encoding with custom Base64Engine instance
  to reduce code duplication.
* more through unittests for Base64Engine.