need overall makefile

[poldrack]

it would be nice to have a guide so that someone could run the entire code base from beginning to end (excepting maybe the parts that rely on a cluster). one approach would be to generate a Makefile - or at least a complete recipe for the user.

[ehbeam]

I've added a plot_main.py script that will generate the plots in the main text figures. The script uses precomputed null/bootstrap distributions and classifier fits so that it can run on a local machine. We can also add a script for the extended data plots, but for now, this is a start!