Added Logarithmic Naive Bayes Learner. (#928)

* test case for zebra problem

* Revert "Merge remote-tracking branch 'upstream/master'"

This reverts commit 5ceab1a27ff73b410200c2f84d22e0a1d79b6fbe, reversing
changes made to 34997e48547a57939d568f170c60e442c2d54db5.

* discarded HEAD changes for merge

* added ensemble_learner jpeg

* updated travis and search

* Added logarithmic learner in nlp_apps (#890)

* Remove zebra tests

* revised Bayes learner explanation

* added missing SimpleReflexAgent from upstream