remove recommendations of pylab

in favor of `%matplotlib`, which does the same thing, excluding imports.

- `%pylab` is gently discouraged, especially in teaching situations, because hiding namespaces confuses new folks.
- `ipython notebook --pylab` is strongly discouraged (and deprecated in IPython 2.0) because its effects are not visible in the notebook, causing great confusion and inconsistent behavior in notebooks depending on how the server was started, with no visible indication about why that may be.