Fix issue #21,support both python 2.7 and python 3.4

[seetea]

Fix issue #21, to make "serving/tensorflow_serving/session_bundle/example/export_half_plus_two.py" and "serving/tensorflow_serving/session_bundle/exporter.py" support both python 2.7 and python 3.4.

[tensorflow-jenkins]

Can one of the admins verify this patch?

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[seetea]

I have signed the Individual CLA. Thanks.

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