plotting.py modified to make it accept numpy arrays

Motivations behind the change
========================
We want to be able to plot, using the plotting facility, th e connection matrix of the network.
Such a connection matrix can be plotted as numpy array.
I propose a modification of utility/plotting.py that permits to plot such an array.

Testing
=====
I have added test_randomness.py in examples. You will be able to test this functionality by running:

    cd PyNN/examples
    python test_randomness.py mock COBA --plot-figure=mock

This will produce the image mock.png, that can be visualized using eog:

    eog mock.png

![mock](https://cloud.githubusercontent.com/assets/5656774/3386205/d9e7c7d2-fc71-11e3-903f-40e7319c6a5b.png)


NumPy also has a matrix class, which is a bit different to array, so perhaps this function should be called `plot_array()` and the argument be `arr`?