Heroku fixes

* simplify instructions for Heroku deploy (if you `git clone` and `cd` first, the `heroku` commands automatically create the remote and also operate on it by default)
* make a convenient init command, invokable via composer, for creating the database fixtures
* add an app.json for/with Heroku button to the README, so people can one-click deploy to Heroku
* change docs index and json data to use dynamic hostname so it works on own deploys
* log to stderr
* add the buildpack as a local dependency so you can `foreman start` (with a `.env` that has a `CLEARDB_DATABASE_URL`)