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

This also changes the `:user` key to `:username` returned from the
connection url hash
Merge pull request #38 from rossta/extend_connection_url

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