Refactor export to ONNX functionality (#258)

Introduced a new utility function to export image-classifiers
to ONNX: export_img_classifier_to_onnx.
The functionality is not new, just refactored.

In the sample application compress_classifier.py added 
--export-onnx as a stand-alone cmd-line flag for specifically exporting 
ONNX models.
This new flag can take an optional argument which is used to name the
exported onnx model file.
The option to export models was removed from the –summary argument.
Now we allow multiple --summary options be called together.

Added a basic test for exporting ONNX.