PostgreSQL & MySQL support

[dinever]

I'm planning to add a button in the README file so that users can easily deploy the application without owning a server.

However, Heroku does not support SQLite for some security reasons. So let's add some support for PostgreSQL which Heroku supports very well.

This shouldn't be hard to achieve as we already have those SQL statements. To switch between SQLite and PostgreSQL/MySQL let's add an environment variable named DATABASE_URL, which indicates the URL of the PostgreSQL/MySQL server. If the environment variable is not set, use SQLite instead.