Supporting Model Subclasses

[jchang10]

I noticed that Model subclasses were not supported. I did a hack in types.py:construct_fields() function to walk the Model's subclasses as well as the Model class itself. It seems to work. It is just a couple lines to modify. Just wanted to note this in case someone else hits this issue.

Thanks for all the work!
Jae

[yfilali]

Would love a pull request if you get a chance, or you can paste the code here with any ideas on a test...

[madama]

@jchang10 would be great if you can share your solution