CLN: move plotting functions out of library

[sglyon]

After a discussion with @jstac, we decided to move the plotting functions in arma.py and ecdf.py out of the main library.

The ones from arma should go into a file examples/arma_plots.py. We can probably do away with the ecdf plot unless anyone objects.

[jstac]

Removed plotting function from ecdf.py and the matplotlab import. Modified numpy_solutions.ipynb file to adjust for this change. Checked lectures and no further changes are required (at least with respect to ecdf). All changes are pushed to master.