base64-decode doesn't exist

[Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai Großjohann)]

I had to replace base64-decode with base64-decode-string and
base64-encode with base64-encode-string in imap.el to make nnimap work
with Emacs 20.4.

kai
-- 
I like BOTH kinds of music.