GNU R fast and URL-safe Base64 encoder and decoder

 In contrast to RFC3548, the 62nd character ("+") is replaced with
 "-", the 63rd character ("/") is replaced with "_". Furthermore, the encoder
 does not fill the string with trailing "=". The resulting encoded strings
 comply to the regular expression pattern "[A-Za-z0-9_-]" and thus are
 safe to use in URLs or for file names.
 The package also comes with a simple base32 encoder/decoder suited for
 case insensitive file systems.