Add fromUri PostgresqlDriver factory

[chances]

I keep my database connection information as an environment variable, postgres URI connection string.

This change lets me easily do this:

PostgresqlDriver driver = new PostgresqlDriver.fromUri(dotenv.env['DATABASE_URL']);

[emilniklas]

Cool, thanks!

[emilniklas]

Will tag a new version when I get the time, cheers

[chances]

That was fast, thanks! 😄