[BioPython] Clustalw multiple alignment

[Jeffrey Chang]

> I don't mind these requirements (obviously :-), but I don't know how
> other people feel about this. 
> 
> But, if no one has any contrary opinions, I would be happy to include the
> clustal stuff I wrote. 

Since nobody objects, and a few people have expressed interest, let's go
ahead and plan to put this in.  Before you do, though, could you run a
quick check to see whether the existing code works under 1.6/2.0?  I'm not
sure if anyone's done this yet.  You could at a minimum run the regression
test suite, and pray it's comprehensive enough to catch problems!  :)

Jeff