[patch] auto dependency removal

[Gustavo Niemeyer]

> unused dependencies. The patch is based on code from Gustavo (thanks!)
> and uses the techniques pioneered by Daniel Burrows in aptitude, all
> the bugs are added by me.

Hey, don't take all the credit. I'm sure some bugs there were
introduced by me as well. :-)

Thanks for the great work Michael, I'll have a look at it and try
it out as soon as possible.

-- 
Gustavo Niemeyer
http://niemeyer.net