Added commandline arguments

I wanted to run a change and see if your interested. I can add them to all the files.

I added the argparse (python2.7 only) module to add a help message

    usage: csv2vw.py [-h] [-l LABEL_INDEX] [-s] input_file output_file

    positional arguments:
      input_file            path to csv input file
      output_file           path to output file

    optional arguments:
      -h, --help            show this help message and exit
      -l LABEL_INDEX, --label_index LABEL_INDEX
                            specify index of label col
      -s, --skip_headers    specify if there are headers in the file - default
                            false


This is obviously not ready to merge but just wanted your thoughts.


Other changes:

* replace tabs with spaces to comply with PEP8
* added string support for the construct line