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

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.