Repo’s git history is too large

@stevemart @devinaconley In the past `.mlmodel` files and `.zip` files full of training data were stored and changed in this repo. This has caused an enormous growth in the git history and has left us with cloning times of upwards of 10 minutes.

I propose we purge the history of these files.

https://help.github.com/articles/removing-sensitive-data-from-a-repository/