Modified version of base64.

URL:http://www.adp-gmbh.ch/cpp/common/base64.html
 
   base64.cpp and base64.h
 
   Copyright (C) 2004-2008 René Nyffenegger
 
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   René Nyffenegger rene.nyffenegger@adp-gmbh.ch

Modified by jmarantz@google.com to:
      - Add a static map and an initializer function
      - add a web-safe variant that uses '-' and '_' in lieu of '+' and '/'
      - change the 'decode' interface to return false if the input contains
        characters not in the expected char-set.
      - removed is_base64 helper function and instead rely on the char_maps.
      - use a static constant for the padding character '='