title: How to implement the predict_proba(X) -equivalent of Scikit-Learn in MLlib
body: <p>python-wise I am preferring .predict_proba(X) instead of .decision_function(X) since it is easier for me to interpret the results. as far as I can see, the latter functionality is already implemented in Spark (well, in version 0.9.2 for example I have to compute the dot product on my own otherwise I get 0 or 1) but the former is not implemented (yet!). what should I do \ how to implement that one in Spark as well? what are the required inputs here and how does the formula look like?</p>