Added support for nn (single layer) in sklearn interface

Adding support for nn to be called from the wrapper.

[DEBUG] Running command: "vw --learning_rate=5.000000 --l2=0.000010 --oaa=10 --nn=4 --passes 10 --cache_file /home/vvkulkarni/vowpal_porpoise/examples/example_sklearn.cache -f /home/vvkulkarni/vowpal_porpoise/examples/example_sklearn.model"
[DEBUG] Running command: "vw --learning_rate=5.000000 --l2=0.000010 --oaa=10 --nn=4 -t -i /home/vvkulkarni/vowpal_porpoise/examples/example_sklearn.model -p /home/vvkulkarni/vowpal_porpoise/examples/example_sklearn.predictionecqcFA"
Confusion Matrix:
[[34  0  0  0  1  0  0  0  0  0]
 [ 0 29  0  0  0  0  0  0  0  7]
 [ 0  0 35  0  0  0  0  0  0  0]
 [ 0  0  0 24  0  4  0  3  6  0]
 [ 0  0  0  0 34  0  0  0  3  0]
 [ 0  0  0  0  0 37  0  0  0  0]
 [ 0  0  0  0  0  0 37  0  0  0]
 [ 0  0  0  1  0  0  0 32  2  1]
 [ 0  1  0  0  0  1  0  1 30  0]
 [ 0  0  0  0  0  2  0  1  3 31]]
0.89717036724
Adding @aboSamoor (as he is interested in this CL too)