Create API for user sign up

[jch254]

• Create a new SLS API based on https://github.com/jch254/serverless-node-dynamodb-api
• Add endpoint to "sign up" taking username, shipsFrom, destinationEmail and store in persistent Key Value store
• Add private function to "set up" monitor for a new user by provisioning a new CloudWatch scheduled 12h event rule with provided data
• Email user after successful setup and outline Discogs Market Monitor and the schedule of emails (including one currently sending)
• Send initial digest

[jch254]

Initial skeleton here, will move TODOs there