Updated README, added comment for mapred_tfidf

Updated the README to show how to run the MapReduce jobs

Added short comment to the main runnable script
Updated environment variables for sampa cluster

there are a few new environment variables needed to run the mapreduce
jobs so I updated hadoop-streaming-env.sh to reflect those changes for
the paths on the sampa server

I didn't know the appropriate path for $NLTK_HOME, so I set it to the
current directory which should work if the scripts and corpus are in the
same directory.