Increasing performance on sparse features using prototyping of zero-valued features

Intermediate work on zero prototypes

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>

Fixing prototype issues

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>

Adding null check to cope with prequential evaluation

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>

Adding null check for estimator request on previously unseen attribute in a given class

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>
Adding comment for smoothing factor + cleaning up imports

Signed-off-by: Olivier Van Laere <vanlaere@yahoo-inc.com>
Merge pull request #113 from ovlaere/zero-prototype

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