Conform to sklearn estimator API

[Jonathan Shor]

Changes required include, at least:

1. .fit(X) returns the class instance, not the labels, to allow chained calls (clf.fit(X).predict(X)).
2. Add .predict(X).
3. Add .predict_proba(X).
   3a. Not sure that we need .predict_log_proba(X)

[Adam Gayoso]

What will .predict_proba(X) return? The voting average or all p-values?

[Adam Gayoso]

I also see a number of issues with the docstring, but I'm assuming we will update those with the 2.0 release.