Transition to Python and Serverless

For fast release, this library was originally written in Node. In order to extend it's functionality, we will be gradually re-writing in Python:
- Better integration with the machine learning community
- Better support for non-image inputs (numpy arrays, GeoTIFF, `sentinelhub-py` to start)
- Replace kes.js with [serverless](https://serverless.com/framework/docs/getting-started/) to reduce our institutional maintenance burden.

We will also add easier configurability for the primary lambda function (`download-and-predict`) at the same time, though this functionality is largely language agnostic.