Support database url for PostgreSQL

For now we have to setup username, password, host and database name, etc:
```rb
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`:
```rb
database PostgreSQL do |db|
db.url = "my_username:password@localhost:5432/my_database_name"
end
```