Modify Dossier::ConnectionUrl to work with arbitrary query string params

Useful for connecting in environment where database connection is specified by url (e.g. Heroku) and the db requires additional configuration, say `ssl_ca` for [Amazon RDS](https://devcenter.heroku.com/articles/amazon_rds)

This also changes the `:user` key to `:username` returned from the connection url hash which I believe to be the more typical key.