Allow controlling the number of cores pyspark uses using the `-c` option  like spark-shell

I couldn't find a way right now to control the number of cores an interactive `pyspark` session uses, which is annoying if more than one person is trying to work on a cluster interactively at once. 

So,

 - Use the python 2.7 stdlib argparse library to pull out the `-c` option
 - Change the bin/pyspark shell script to pass all arguments to the python script instead of allowing the python interpreter program to parse any of them.