Make PNG summary compatible with latest SummaryGraph class changes (#7)

* Modify 'create_png' to use the correct data structures (dicts instead
  lists, etc.)
* Handle case where an op was called not from a module. This relates to:
  * ONNX->"User-Friendly" name conversion to account for cases where
  * Detection of existing op with same name
  In both cases use the ONNX op type in addition to the op name
* Return an "empty" shape instead of None when ONNX couldn't infer
  a parameter's shape
* Expose option of PNG summary with parameters to user