Add some (broken) base64 decoding code

This doesn't work so it's commented out.  I don't really want to deal
with it now, as I don't have any important base64 stuff to decode, but
I figured I'd just leave the code there so it doesn't get lost.
Implement "mhng-install"

This script sets up your $HOME/.mhng directory to contain what MHng
requires.  This greatly simplifies the new MHng code because I don't
have to worry about fixing up the database as it goes.

Note that this script is far from done: it should really ask the user
for a bunch of information.  That said, for now I think it's good
enough: the stuff I want to add isn't supported by the old code so
I'll need to eliminate all of that first.