Compatibility fix and changes in 'main'

* Replace 'xrange' for 'range' and using 'print' as a function.
* Main now prints all combinations of algorithms and feature selection.
Log probability of Normal Distribution

Computation of the probabilities in our Naive Bayes Algorithm is now
made with the log of the Normal Distribution's pdf, helping with
underflow. 'Division by zero' warnings alse were removed.
Merge pull request #2 from gabriel-almeida/master

Log of Gaussian Distribution