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 `DATABASE_URL` and expects 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.

If you want to test this out (that would be awesome!) be sure to rebase this branch in to your local master branch, becase heroku only allows deploy from that branch.

The following command should suffice:

    git rebase heroku-integration

The steps to deploy to heroku are as follow:

    heroku create --stack cedar
    git remote add heroku (URL from above)
    heroku addons:add heroku-postgresql:dev
    heroku config:set NODE_ENV=production
    git push heroku 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.