Test tutorial notebooks with Python 3

[cweill]

Currently they are only tested with Python 2.7.

[smyrbdr]

Just tested the costumizing_adanet tutorial on MNIST dataset with Python 3.

The only thing I needed to change was the line (images=features.vaules()[0]) in build_subnetwork function with the following line;
images = tf.to_float(features[FEATURES_KEY])

Here is the result for those who want to check; https://github.com/smyrbdr/AdaNet-on-MNIST