Don't use pylab

It's OK to require that users start ipython notebooks with `ipython notebook --matplotlib=inline`, but we should avoid encouraging `ipython notebook --pylab=inline`:  http://carreau.github.io/posts/10-No-PyLab-Thanks.ipynb.html

I haven't checked the tutorials, but we should explicitly do something like:

```
from matplotlib import pyplot as plt
plt.plot(...)
```
to encourage good namespace behavior.  We could also include `%matplotlib inline` in the startup for each notebook, but that's noise.