a python implmentation of label_image (#9261)

* a python implmentation of label_image

A quick implementation of tensorflow/examples/label_image/main.cc
in python.

With data file described in [1], we can we get reasonable results

$ python3 ./tensorflow/examples/label_image/label_image.py
military uniform 0.834306
mortarboard 0.0218692
academic gown 0.0103579
pickelhaube 0.00800814
bulletproof vest 0.00535088

[1] https://www.tensorflow.org/tutorials/image_recognition

* fix the gif decoding problem and add bmp decoding

* Add from __future__ as per style guide.