This module is a perl version of python's URL-safe base64 encoder
/ decoder.

When embedding binary data in URL, it is preferable to use base64
encoding. However, two characters ('+' and '/') used in the standard
base64 encoding have special meanings in URLs, often leading to
re-encoding with URL-encoding, or worse, interoperability problems.

To overcome the problem, the module provides a variation of base64
codec compatible with python's urlsafe_b64encode / urlsafe_b64decode.

Modification rules from base64:

    use '-' and '_' instead of '+' and '/'
    no line feeds
    no trailing equals (=)