Implements base 64 encode and decode, see RFC2045.
Here follows type definitions that are used by more than once this module.
string() = list of ASCII characters
Types:
PlainASCII = string()
Base64 = string()
Encodes a plain ASCII string into base64.
Types:
PlainASCII = string()
Base64 = string()
Decodes an base64 encoded string to plain ASCII.