Merge pull request #25 from j0hn/develop

Completed some docstrings
Fixed bug in naivebayes (adding __len__ makes "if x:" work)
Unused variable
Bug fix, you cannot have target in tree
Merge pull request #26 from rafacarrascosa/develop

bug fixes: naive bayes empty training and decision tree splitting by target
Merge branch 'develop' of https://github.com/fisadev/simpleai into develop