SciUnit interface for the Potjans2014 model

[lungsi]

I have made two changes:

1. in plotting.py I have added
   import matplotlib matplotlib.use('Agg')

2. I have created a validation_microcircuit.py which contains the class Potjans2014Microcircuit. The class is mostly based on microcircuit.py with some modifications. To use it with sciunit you just need to uncomment lines 27-28 and 33-34 and comment line 36.

3. To use just
   from validation_microcircuit import Potjans2014Microcircuit as pm
model = pm()
model.produce_spiketrain()

[coveralls]

Coverage decreased (-11.8%) to 42.347% when pulling 016edd3 on lungsi:validation into 3e336ef on NeuralEnsemble:master.