Another deb822 branch

[John Wright]

Dato, Dann,

After writing six or eight dict methods to make sure everything was
handled correctly in Deb822Dict, I realized the advantage of dato's
original approach using UserDict.DictMixin.  But, I made a rather
hackish use of Python's multiple inheritance so that Deb822Dict objects
appear to be instances of dict, and I'd like to get your feedback on
this.  (See the commit log and comment in the file for a more complete
description.)

If the extra dict inheritance is problematic, I'm ok with removing it.
I might be a little too stuck on this thing being a "real" dict
instance. :)

The branch is here:
  http://free.linux.hp.com/~jswright/deb822/apt_pkg+case_insensitive+dictmixin

Thanks!
John