Completed a few TODOs

Hi Jake!

I just completed a lot of TODOs:

* updated the docstrings for the fitting and prediction methods to match sklearn style comments.
* ensured that there are only 2 classes
* mapped class predictions to classes_
* fixed the ugly try except pass in the fit methods
* Renamed the precomputed_kernel constructor parameter and the underlying fit methods to make the meaning of this parameter easier to understand.
* and more...

For the usage example, I think that your example is good. You are just missing:
pu_clf.fit(x,y)
pu_clf.predict(x)
This corresponds to a prediction on the training set, but it still gives an intuition on how to use POSOnly.

I will write a proper example to show that POSOnly really helps when positive examples are hidden in the unlabeled examples. A few months ago, I wrote an example which could probably be adapted easily (see https://github.com/aldro61/pu-learning/blob/master/src/tests/breastCancer.py)

Let me know if you have any comments or suggestions :)

Alex