Add Gaussian Naive Bayes classifier for spot checking

[bkowshik]

We currently spot-check the following models:

• LogisticRegression
• DecisionTreeClassifier
• KNeighborsClassifier
• SVC
• RandomForestClassifier
• GradientBoostingClassifier

Let's add Gaussian Naive Bayes (GaussianNB) to the mix too.

• http://scikit-learn.org/stable/modules/generated/sklearn.naive_bayes.GaussianNB.html