Enhance model by enabling multiple recurrent layers

[gb96]

The model creation functions defined in nn_utils.network_utils include an optional parameter num_recurrent_units=1

None of the calls to these functions from train.py and generate.py specify a value for this parameter, so by default there is only ever 1 hidden layer.

Add a parameter to config.nn_config for the number of hidden layers and then use the value of that parameter wherever the model is created.