Add SGD

SGD Classifier and Regressor added

The default parameters are:

```python
'alpha': [0.000001, 0.00001, 0.0001, 0.001, 0.01], 'penalty': ['l1', 'l2', 'elasticnet']
```
You should just add `SGDClassifier` and `SGDRegressor` to the same `import` statement that grabs `ElasticNet` and the others from `sklearn.linear_model`.