Made nef_summary example notebook

I made a little set of plots to summarize the NEF for the Nengo paper, which seemed appropriate as an example notebook. [Check it out](http://nbviewer.ipython.org/urls/raw.github.com/ctn-waterloo/nengo/nef_summary/examples/nef_summary.ipynb)!

This PR also fixes a few bugs that came up when I did some hacky things. Also removed a print statement from model.py.