Add more preprocessing pipeline operators

[rhiever]

Add the following preprocessing pipeline operators:

• StandardScaler: http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.StandardScaler.html#sklearn.preprocessing.StandardScaler
• RobustScaler: http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.RobustScaler.html#sklearn.preprocessing.RobustScaler
• PolynomialFeatures: http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.PolynomialFeatures.html#sklearn.preprocessing.PolynomialFeatures
• PCA: http://scikit-learn.org/stable/modules/generated/sklearn.decomposition.PCA.html#sklearn.decomposition.PCA

[rhiever]

Note to self: Investigate this more.

self._select_percentile(self._polynomial_features(training_testing_data), 10)

Results in the warning:

UserWarning: Features [0] are constant.

Same for passing the results of _polynomial_features() to any feature selection function.

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Also, still need to implement export functionality for the new preprocessing operators.