Implement image similarity

Branch: `image_similarity`

### Task at Hand
- Do some research on image similarity and what methods we could use to compare how similar two images are. We would be using this method to see how close a user drawing is to the sketch that was created by our program. List at least three different options that you are considering in the Design Document.
- Communicate to us when this step is done on slack so we can review the options you've chosen!
- Choose the best option and implement `image_similarity` function. Test it to make sure it works.