Build this repo in Docker so it can be developed locally

[mbernier]

Issue Summary

It is really nice to be able to develop locally inside a docker container! Please dockerize this repo!

• You can probably use the Dockerfile for sendgrid-python
• Please setup docker-compose to use a local volume for the library files (this is currently different from sendgrid-python)
• Please add information to the README and USAGE files about how to startup the container