use .env file for DATABASE_URL if present

This uses the `dotenv` module to read environment variables from a `.env`, if present, allowing the user to specify the database creds via this file locally.

This simplifies e.g. writing apps that get deployed to Heroku and use db-migrate.

* My server code can always depend on having the DATABASE_URL environment variable set. Locally via `.env` file, and in production set on the server
* db-migrate will work in both environments cleanly this way
* I don't have to code my server to be aware of the database.json file or ensure that files are there locally / in production.

In short, i feel using this makes handling different environments a little more clean.

I dont have tests as I dont have mysql running locally but could drop some in if you have some suggestions.