Improving performance of Naive Bayes using a prototype for zero valued features

In the current version of the Naive Bayes classifier, we iterate over each attribute, even though this might be very sparse. This is very inefficient.

To cope with this, we implement the following computation (already down to this specific computation)
![nb_prototype](https://cloud.githubusercontent.com/assets/2300860/4955260/fd23ffc2-6693-11e4-81e4-be9f92e71933.png)

This pull request at the same time fixes a nullpointer using NB and prequential evaluation, where there was no classmass assigned yet, while it was being requested by the prequential phase. The classmass is no longer used and thus the bug fixed.