Allow controlling the number of cores pyspark uses using the `-c` option, like spark-shell.
 
 - Turns out there isn't actually 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. 
 - Use the python 2.7 stdlib argparse library to pull out the -c option
 - This requires changing 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.
Merge branch 'pyspark_cores'