Support Python 3, Newer TensorFlow

[jamesmcclain]

This changeset does a few things. It

• Adds support for Python 3. Python 2 should still continue to work as before.
• Adds support for newer TensorFlow versions. This change is not compatible with older versions. That can be changed in a subsequent commit if there is interest.
• Addresses a minor typo in the README.

train.py, evaluate.py, and inference.py all appear to work on top of the tensorflow/tensorflow:1.12.0-rc0-gpu-py3 docker image with these changes.

[jamesmcclain]

This largely duplicates #140, can probably be closed.