Added data preprocessing script

description: This script separates the debate corpora into conversation
turns and then saves the data in a format that we can use more easily
for topic modeling and prediction (one directory of text files for
mallet, and currently two other files, for training and testing, with
metadata about each document).

input (specified with command line args):
    * output_name: prefix of the output file names (seeting this to
    "debates" would create two files and one directory: debates.train,
    debates.test, debates/
    * corpora_info: this argument specifies a json file with info specific
    to each corpus (e.g., 'input_example.json')

usage:
    python format.py output_name corpora_info