Add tflite exporter

Does not fully work yet:
Some of the operators in the model are not supported by the standard TensorFlow Lite runtime. If you have a custom implementation for them you can disable this error with --allow_custom_ops, or by setting allow_custom_ops=True when calling tf.contrib.lite.TFLiteConverter(). Here is a list of operators for which you will need custom implementations: DecodeBase64, DecodeJpeg, EncodeJpeg, Enter, Exit, LoopCond, Merge, Range, ReverseV2, Switch, TensorArrayGatherV3, TensorArrayReadV3, TensorArrayScatterV3, TensorArraySizeV3, TensorArrayV3, TensorArrayWriteV3