Support database url for PostgreSQL

[dzhlobo]

For now we have to setup username, password, host and database name, etc:

  database PostgreSQL do |db|
    db.name               = "database_name"
    db.username           = "username"
    db.password           = "password"
    db.host               = "localhost"
    db.port               = 5432
  end

I suggest to add one option -- url. Which would look like a string username:password@localhost:5432/database_name:

  database PostgreSQL do |db|
    db.url = "my_username:password@localhost:5432/my_database_name"
  end