reduce.py: add documentation (comments)
log crunch example Makefile: use Hadoop 1.2.1

I just tried walking through my examples today and found that the latest
stable Hadoop is 1.2.1. Everything seems to work fine under 1.2.1, so
I updated this Makefile to reflect that. I'll update the handout/speaker
notes in the slideshow, too.
slideshow: update, add handout content/speaker notes
Makefile: clean up dumbo-hadoop detritus