cleaned up commit 327f5de for pull request

* mapred_tfidf.py:
  * switched argument parsing to argparse.
  * if MapReduce will overwrite directories, users are now asked whether they want to continue.
  * added '--force' flag to automatically force directory overwrites.
  * better error handling.

* map_reduce_utils:
  * 'numeric' words such as '14th' are filtered out.

* mappers/reducers:
  * added documentation for each script, removed unneeded conditionals, added helper methods for print formatting.
  * word_count_red no longer has to re-read each file to see how long it is.
  * cos_sim_map now ensures that two documents are sent to the same reducer regardless of which order they arrive in.
  * removed old comments that didn't apply any more.