Heroku integration

Initial changes to be able to easily deploy HackFlowy to heroku (for quick testing, etc).

- Added new configuration option for `database.json` and `(production|development).json`. The option is `use_env_variable` and the value should be a environment variable name, ie `DATABASE_URL`, and the value a database connection string, ie: `postgres://user:password@example.com:5432/database`. The old format still works.

- Added a script section to `package.json` to execute all the scripts necessary such as migrations and so on after executing `npm install`. So may be the readme install section is a bit redundant now.

The steps to deploy to heroku are as follow:

    heroku create --stack cedar
    heroku addons:add heroku-postgresql:dev
    heroku config:set NODE_ENV=production
    git push heroku heroku-integration:master

You'll need heroku toolbelt installed. See https://devcenter.heroku.com/articles/heroku-command

There is a live heroku instance running at http://agile-plains-2755.herokuapp.com/

Hope somebody could take a look at this PR and report any issue/question etc.