Create izhikevich_net.py

Izhikevich network.

This script re-implements the network model presented in [1]
using pyNN v-0.7.5 [2].

References

    [1] Izhikevich, Simple Model of Spiking Neurons,
        IEEE Transactions on Neural Networks (2003)

    [2] Davison et al., PyNN: A Common Interface for Neuronal Network
        Simulators, Frontiers in neuroinformatics (2009)