Added predict_proba to logistic regression and added the appropriate unit test

[cbonnett]

Well, the title says it all really.
Thought this might be useful.

[ilblackdragon]

I think it would be better to move the _session.run part to def _predict function to have only once and share between predict and predict_proba

[cbonnett]

I see what you mean, and would make sense. Not sure how to technically implement it though.

[ilblackdragon]

Something like this:

def _predict(self, X):
    return self._session.run(self._model.predictions, feed_dict={ .. })

def predict(self, X):
    pred = self._predict(X)
    if self.n_classes < 2:
       return pred
    return pred.argmax(axis=1)

def predict_proba(self, X):
     return self._predict(X)

[ilblackdragon]

I'll merge yours in and will implement the suggestion from above as I'm also adding scoring with mini batches and streaming and will close this PR. Thanks!