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']
```
Please ensure that all lines are shorter than 80 characters (it's really supposed to be 79, but a lot of the code in SKLL uses 80 as the limit instead).