WIP: explicitly mark all base classes as ABC with abstractmethod inits
Add concrete __init__ for LinearSVM
Add concrete implementation for SGDClassifier
BUG: implement concrete __init__ for SGDRegressor
BUG: tests were raising the DataConversionWarning

It appears that when warnings are raised once, they are not reraised
in the tests, thus triggering a failure for the test checking for these
warnings
Merge branch 'pr_2304'

Conflicts:
	sklearn/linear_model/stochastic_gradient.py